Activaction groups are sub-environment wihtin a job. In one you can have more than one activation group. For OPM jobs, there is only one: Default Activaction Group. ILE programas runs in a different activaction group. Therefore, you could have more than one activaction group in a job if you’re running ILE programs.
The purpose is to separate different applications in order they have their own environment, specially when they share access to one or more files.
Then the commands OVRDBF, OPNQRYF, COMMIT and others have the ACTGRP parameter in order to be defined to act in a certain environment. This way you can decide if Account Receivable program will use same file cursor that G/L did for the master account file. Depending on your needs you set the rules of access for every file.
For doing that you have to deal with concepts and parameters like the type of activaction groups (*NEW, DAG, named activaction group), OVRDBF activaction group scope parameter (*JOB, *ACTGRPDEF, *CALLVL), the Open Data Path, etc.
It’s just a brief explanation and you have to dig more about ILE concepts to get full knowledge of all this stuff.