Here's a scenario: We have a .fla file with two movie clips named "MC1"
and "MC2" in the "Scene 1" timeline. Each of these
also contain two movie clips called "MC1a", "MC1b"
for the first and "MC2a" and "MC2b" for the second.

From within "MC1a" you can reference "MC2" like
this:
/MC2
or _level0/MC2
"_level0" refers to the first scene location regardless of
its name. To reference the second scene, it would be "_level1".
To reference "MC2a" from within "MC1a":
/MC2/MC2a
or
_level0/MC2/MC2a
To reference "MC1b" from "MC1a" use any of these:
_level0/MC1/MC1b
../MC1b
or
/MC1/MC1b
Accessing loaded movies loaded into "layer1" with the "Load
Movie" action are referenced by:
_layer1
You can access all the elements and variables in the loaded movie the
same way as normal with this in the front of the target string.
Next
Tutorial: Symbol Combinations, Part
II (Grouping Symbols)