What is value substitution in SAP Process Integration?

Value substitution is an Adapter Specific Message Attribute (ASMA) provided by SAP Process Integration.

As an example, you need to read a text file by SAP PI and process that data to output another file to a third-party system. In this scenario, you need to get the input file name to output the same file name to the output file. Value substitution in file adapters can be used for this purpose.