HomeNW operator identify other as HPLMN?

Spec 23122 Rel 07: It is possible for the home network operator to identify alternative Network IDs as the HPLMN.

Question:
1. Can You please tell me practically when Home N/W operator identify the other N/W operator PLMN as HPLMN?
2. If MS found alternative PLMN ID & suppose if its HPLMN as explained in spec 23122, based on what MS will recognize that as a HPLMN.??