In PUBM, it states, it is like MSGM.
(5) MSGM (stackable)
bind msgm <flags> <mask> <proc>
procname <nick> <user@host> <handle> <text>
Description: matches the entire line of text from a /msg with the
mask. This is useful for binding Tcl procs to words or phrases
spoken anywhere within a line of text.
Module: server
Thus, PUBM is the channel triggered version.
This bind allows you to match whole lines, rather than just the first word, based on a trigger.
IE, take this line of incoming text
If you use a mask of "* http://*", in PUBM or MSGM, it will trigger on any line (based on the bind, and where the message is destined) that has a URL, begining with http://.
Whereas PUB and MSG binds, expect a ketword, that is the first word in a line of text.