Validation and Substitution:
Validations and substitutions are used to check the conditions while posting a document in sap system.
The step of Validation is.
Setting up the FI Validations:
Substitution permits the customer-specific enhancement (substitution) of certain field contents.
Substitution consists of several steps (up to 999 are possible), each with two parts:
Substitutions are not supported by the document parking feature. Substitutions can only be made with posting transactions after turning parked documents into accounting documents.