Audio & Video Codec for Video

I have a few questions on use of Gspot which I can not find in Forum
or NG. Hope some experts can answer them for me.
1. In ViewInstalled CodecsVideo, my list show 3 different Type i.e.
DMO/DSH/VFW.
a)What is the difference among them and when do you use one over
the other?
2. Under each Type e.g. DSH, I see 2 different DIVX codec listed
(Intervideo Inc, DivXNetworks),yet when I render a Divx 5.0 AVI clip,
the Video Path indicated (S) -- AVI Splitter -- ffdshow MPEG-4 Video
Decoder -- (R)
a) Why ffdshow is used over Intervideo or DivXNetwork even it is
not under the DIVX format?
b) Similarly for the audio portion, it show that it is a
ac3(0x2000) Dolby Labs. Inc format and listed 2 compatible codecs(
AC-3 ACM Decompressor & ffdshow Audio Decoder). The Audio Path
indicated S) -- AVI Splitter -- ACM Wrapper -- (R). Why ACM Wrapper
and one of the 2 compatible codecs? What is ACM Wrapper as it is not
even listed in the compatible list?
3. How does Gspot or any video player chose what codec to use to
render a AVI clip if more than 2 or more codecs of same 4cc are
installed? Is there anyway to put/configure what particular codec to
use eg how can I chose DivNetwork codec over ffdshow in 2 above?
Thank You for your time and Help