what is composite EXA operations for?

[cckuo]

Dear X friends:
Recently I find there is a option can be added in xorg.conf
Section "Extensions"
			Option "Composite" "Enable"
EndSection
What is that used for, does it has any relation with EXA composite
functions?
If it really is, does the composite control the pixmaps in EXA, since I saw
it somewhere in the source code of EXA? 

Appreciate your help in advance,
Sincerely Yours,
cckuo