The RMF Step and task where the Information System (include system boundary) is described and documented in the Security Plan Response:

RMF Step 1, Task 1 typically involves identifying the Authorizing Official and key stakeholders.

RMF Step 1, Task 3 focuses on identifying and documenting common controls and their providers.

According to NIST RMF, Step 1 (Prepare) includes Task 2, which specifically mandates describing the information system, defining its boundary, and documenting this information in the security plan. This task establishes the foundational scope for all subsequent RMF activities.

RMF Step 1, Task 4 generally involves identifying and documenting organizational roles related to the RMF process.