BCPii is used by the SOLVE:Operations Automation Hardware Interface Service (HIS) to communicate with System Z's Hardware Management Console (HMC).
This link takes the reader to an IBM BCPii SHARE publication that discusses BCPii's setup and APIs in good detail:
www-03.ibm.com/support/techdocs/atsmastr.nsf/5cb5ed706d254a8186256c71006d2e0a/2f3c49c0a320bdca862576f5005fb64d/$FILE/SFM%20with%20BCPii.PDF
For further information about the SOLVE:Operations Automation Hardware Interface Service (HIS) please see publication Hardware Interface Service Component Guide Version 1.0 (part of the SOLVE:Operations Automation r11.9 bookshelf).
Note: BCPii is IBM's Base Control Program internal interface.