How EHPLMN derived from IMSI???

Spec 31.102 Rel 07: This EF contains the coding for n EHPLMNs. The usage of EHPLMN is defined in TS 23.122 [31]. This data field may
contain the HPLMN code derived from the IMSI as an EHPLMN entry.

Question:
1. How the EHPLMN derived.?
2. If I am roaming to the place where my home operator-A doesnt have a service but has a tie up with other operator-B. But the operator-B is not included in EHPLMN list. Will the MS only latch & avail service. Or at same case, it can be included in EHPLMN list also.
I think it may not be included since EF-EHPLMN update access condition is ADM