[Samba] SIDs and UIDs and RIDs - Oh My!

[Moondance Foxmarnick]

I'm trying to grasp pg. 154 of the "Official SAMBA-3" book by Terpstra and
Vernooij and I'm just missing a critical networking concept.

I understand that SIDs are the numerical identification of a user for the
Windows world.

I understand that UIDs are the equivalent for the *nix world.

But what the @$@! is a Relative IDentifier (RID)?!?

On page 153 the command to map a windows group to a *nix group - no mention
of RIDs.

Then on 154 it is stressed that under no circumstances should your *nix
groups or users trod on window's assigned RIDs for Domain Admins, Domian
Users, et. all. Another example of groupmap - oh look it lists a RID?

No mention as to where a RID comes from or can be viewed. 

Do they mean that I can't have a user in Fedora that is 500? Isn't that a
UID? Is a UID a RID? I've used Fedora for a year now and have never typed a
RID modifying command.

I'm sure this is just so basic. But I don't know it and can't find it and
it's critical to understand it.

T.I.A.