You control the availability check using general and SD-specific control features. The following control elements need to be maintained in Customizing and in the material master record:_ Strategy groupThe allowed planning strategies (main strategy and further possible strategies) are combined in the strategy group. As of Release 3.0, the strategy group is specified in the material master record in the MRP 1 Screen. In Customizing, strategy groups are assigned, dependent on plant, to MRP groups. If the strategy group is missing in the material master record, it is determined on the basis of the MRP group, if it has been  maintained.  Up to Release 3.0, the strategy group is determined on the basis of the MRP groupMRP group : The MRP group combines materials from the point of view of material requirements  planning. This enables you to control planning for these materials in a particular way using, for example, the strategy group, consumption mode and planning period. You enter the MRP group in the material master in the MRP 1 screen. The strategy group is determined from the MRP group.Planning Strategy : The planning strategy specifies the requirements type for planning and customer requirements. This represents the decisive control feature for the interaction between Production Planning and Sales and distribution.MRP type and item category : If no requirements type is found using the planning strategy, the system tries todetermine a corresponding requirements type on the basis of the MRP type and the item category. Until 3.0C, determination of the requirements type via planning strategy has taken priority. This is not always the best option, however, as the following example will show. A requirements type is determined for a material, which causes availability to be checked against planning. In consignment stock processing, however, availability should be checked against stock. Until 3.0C the availability check had to be deactivated in these cases. As of Release 3.0C, you can determine how the requirements type is to be determined for each transaction by maintaining the Source field in Determination of requirement types using transaction in Customizing.Requirements type : The various requirements are identified by their requirements type. The requirements type refers to the requirements class and its control features.Requirements Class : The requirements class contains all control features for planning such as relevance forplanning, requirements planning strategy and requirements consumption strategy. In addition, it is specified at a global level whether an availability check is to take place for the material in the sales and distribution documents on the basis of the ATP quantity (ATP = available to promise) and whether requirements are to be passed on. A finerdegree of control can be obtained for sales documents using the schedule line category.

Control Features Specific to Sales and Distribution

The following SD-specific control features need to be maintained in Customizing:

Checking group : The checking group controls whether the system is to create indivdual or collective requirements in sales and shipping processing. In addition, a material block for the availability check with transfer of requirements can be set here. The checking group can also be used to deactivate the availability check. This option was created especially for the assembly order so that when the bill of material is exploded in the assembly order, the individual components, if necessary, can be classified as non-critical parts as far as procurement is concerned. The checking group specifies in combination with the checking rule the scope of the availability check. It is proposed in the material master record on the basis of the material type and the plant, and copied into the sales and distribution documents.

Checking Rule : You use the checking rule to control the scope of the availability check for each transaction in sales and distribution. You also specify whether the check should be carried out including or excluding replenishment lead time. The individual checking rules define by transaction, which stock and inward and outward movement of goods should be taken into account for the availability check.Schedule line category : You can control with the schedule line category whether an availability check and transfer of requirements should be carried out in the sales documents. The possible settings for this at schedule line level are dependent on the settings in the requirements class which is determined from the requirements type of the material.Delivery item category : The delivery item category can be used to control whether an availability check takes place in deliveries.

Prerequisites

An availability check can only be carried out if the following prerequisites have been fulfilled:

• The control elements described above for the availability check must be maintained in Customizing for Sales and the relevant assignments made to the sales transactions.
• The availability check must be switched on at requirements class level and – for the availability check in the sales documents – at schedule line category level
• A requirements type must exist by which the requirements class can be found. 
• A plant must be defined. It can either be proposed from the customer or material master record or can be entered manually in the document.
• A checking group must be defined in the material master record on the Sales/plant data screen in the Availability check field.

Global and Fine Control in Customizing

At the global requirements class level, the availability check can only be switched on in combination with the transfer of requirements. At schedule line level, the settings are proposed from the requirements class. If the availability check is switched on at requirements class level, it can be switched off at schedule line level. However, you cannot switch on the availability check at schedule line level, if it is switched off at requirements class level. You can make this setting at schedule line level. But the system ignores it and the setting for the requirements class applies. Also, at schedule line level when the availability check is switched on, the transfer of requirements can be switched off. For example, this makes sense for inquiries or sales information. The settings specific to schedule lines for performing an availability check are only relevant for sales documents. In the shipping documents, the settings from the requirements class and the delivery item category are used. As with the schedule line category, the availability check can be switched off as required in the delivery item category.

Blocking Logic

There are two blocking concepts:_ Material blockIn Customizing, the Define material block for other users, step is used to determine, by checking group and transaction (order/delivery), whether a material master record should be blocked for other users during an availability check. The block is set at material-plant level and remains in place until you save the sales document._ Material block with quantity transferFor performance-related reasons, the only time when it makes sense to set the material block with quantity transfer is when it is common for several users to work on the same material simultaneously. If this is not the case in your organization, you can work with the conventional blocking logic at material-plant level. In Customizing, the material block can be set with quantity transfer under Define checking group. If this block is active, the material is blocked only for the duration of the availability check when you create or change a sales and distribution document.Thus, it is possible for several users to process the same material simultaneously in various transactions without blocking each other. During the availability check, the material being used is blocked exclusively in the current plant as in a conventional block. 

For more details, please refer to this documentation from SAP.

Availability Check