It is assigned for MW15 in your pricing procedure.
Requirement 21 is for "Invoice list control".
This is an example of a pricing requirement. This requirement is met if the document is relevant for invoice lists and the document is either a billing document, a credit / debit memo request, an invoice list, or a return. This requirement can be assigned to condition types in the pricing procedure that are only relevant for invoice lists. This requirement is only available for sales pricing.
This requirement is used for the invoice lists.
One of the example is:
Suppose a company creates invoice lists for their customers which include the billing documents for a payer for a given month. Within the invoice list, the payer earns an additional factoring discount. Within UK, this discount is taxed at a fixed rate of 17.5%. To recognize this tax, the user adds the R/3 delivered condition type MW15 to their pricing procedure and assigns it pricing requirement '21' so that it will only be applied in an invoice list process.It will only come in this invoice lists.
Its effect can be seen in invoice lists.