This article documents the minimum requirements of what must be in the STEPLIB ddname of the startup JCL for an IDMS CV.
Release : All supported releases.
Component : CA IDMS
If you have AllowUserKeyCSA(NO) set in your LPAR, then the requirements of what must be in STEPLIB are documented at Storage Key Considerations for z/OS CSA Subpools.
If you have AllowUserKeyCSA(YES) set in your LPAR and you therefore do not have STEP=Y in your CV startup parameters, then the only load module which must be in STEPLIB is RHDCOMVS (and an alias if one is being used).
Regardless of the AllowUserKeyCSA setting, IDMSUXIT will be loaded from STEPLIB if it there but it does not have to be.
Aside from the above considerations, IDMS will load everything from CDMSLIB.
It is recommended that you put CAGJLOAD in STEPLIB but the above is the absolute mininum of what is necessary.