[STUMP] what is a head?

[Joel Fernandes]

I have small question about what the term 'head' means.
ok, so we have a x11 'display' that has multiple screens (each one
being a crt?) and each screen having multiple heads, each head in turn
having multiple mode-lines. Now why would a single screen have more
than one mode-line? Also what exactly is a head and why do screens
which are on their own crts have multiple such heads?

I've tried hard to answer these question by looking up stumpwm and clx
docs and pretty much everywhere else.
Thanks!
Joel