What could we put into Value element in CAML?

Question

First sorry for my English.

I'm very confuse about querying ListItem using CAML.Look like the Value element could contain many different content.

For example,When we query a field that is type of DateTime, It can be <Value Type="DateTime"><Today Offset="20" /></Value>. Or a Lookup field, <FieldRef Name='Meeting_Set' LookupId='true'/><Value Type='Lookup'>1</Value>. And I saw somebody write like this <Value><ProjectProperty Select="ID" /></Value>

MSDN says "Numerous" element can be Value's child.There are no references to tell me what could I put into Value element.

I need help.