This command is used in order to ask another user to become your partner for a bughouse match. The other user will need to accept or decline the offer. If the other user accepts the partner offer, the two of you will then be able to hold bughouse matches against other partnerships and the two of you can be identified using the "who" command (namely by "who B") as being available for bughouse matches.
See also: accept, bughouse, decline, pending, who