How does one answer prompt "Select gaining space boundary"?

This may be obvious to old hands, but I have the need to merge several
blocks and remove the lines of their common boundaries. I am stuck at
the point where the command prompt is "Select gaining space boundary:"
I would expect to be able to just click on the block to select it, but
the odd little square cursor doesn't do anything, and everything I
have tried entering to specify the boundary doesn't get accepted so
that I can get the expected prompt for the other space. Can some kind
person please tell me what to enter for the prompts? Please reply by
email, since I don't monitor this group regularly. It would also help
to point me to some online documentation for this kind of thing. The
help files are not sufficiently specific.

I am using AutoCAD 14 Architectural for a client who doesn't have
documentation.