What are substitutions and validations in SAP ? What is the precedent?

Validations are used to check settings and return a message if the prerequisite check condition is met. Substitutions are similar to validations; they actually replace and fill in field values behind the scenes without the user’s knowledge unlike validations that create on-screen messages to the user.

Click here to go to Next Chapter. 41. What is a controlling area in SAP FICO ?