Many complex constructions frequently have several exits with the
same name. To tell these exits apart, you tag them with a unique
blurb at the end of the exit name. Take for example, two exits,
called "Pull Lever", which will do different things depending on
an item the player has. Without tags, to change a something on
one of the exits, you'll have to use the object # (the dbref) to
refer to the correct exit.
To tag these exits you could:
call the first exit: "Pull Lever;pull;lever;PL;Lever.1",
and call the other: "Pull Lever;pull;lever;PL;Lever.2"
You can then refer to each as Lever.1 and Lever.2, and say (for
example) "@lock lever.1 = The magic ring" and you'll know you
modified the correct exit. Don't be too obvious with your tag
names though, or explorers will use them too. :-)