What purpose is for using of Application Data Service during the migration

Hello!

We are using Siebel 19.6 and we are trying to run a new deployment by using Siebel Migration Application. It is clear what different Migration Resources (i.e. like Schema Service, RR Data service, etc.) mean and are used for, however it is not clear what purpose the "Application Data Service" and "Application Data Service With Transformation" could be used for?

By using the "Metadata and Files" link we can navigate to "Application Data Rule Expression View" in the application, where some rules for initial data export and import could be defined. When we create a new record and try to select the custom table, which we need to migrate data from, it is not possible to select it from the picklist as it is not available there. the picklist has a constrain on tables beginning with "S_", however our custome table begins with CX_.