Hi Alex,

1.
From what I understand, the cxt-def (Context Definitions) provide metadata (mainly structure information from what I understand) to display to the user and also to help in TM matching.
Have a look at the IContextInfo interface:
producthelp.sdl.com/.../f13808da-c68e-7a28-2c80-c280e1887190.htm

There are two types of contexts.
- TM match contexts (ones that start with x-tm)
and
- Standard contexts for non-TM matching contexts (ones that start with sdl:)

For TM match contexts, there are 3 purposes:
Match: for contexts that are used when looking up in a TM
Informational: for contexts that provide translators with information
Location: for contexts that are used to mark a document location

2. Here is a list of tm match contexts:
x-tm-table-heading
x-tm-table-cell
x-tm-heading
x-tm-header-footer
x-tm-listitem
x-tm-tag
x-tm-label
x-tm-index-entry
x-tm-length-info

And a list of standard contexts:
sdl:address
sdl:block-quote
sdl:body
sdl:bookmark-name
sdl:bookmark-reference
sdl:button
sdl:callout
sdl:caption
sdl:cdata
sdl:cell
sdl:chartsheet
sdl:checkbox
sdl:cite
sdl:code
sdl:combobox
sdl:comment
sdl:content-flow
sdl:copyright
sdl:create-index
sdl:create-table-of-authorities
sdl:create-table-of-contents
sdl:dialog
sdl:dialogs
and the list goes on (the list is huge).
If you want to see them all look in Sdl.FileTypeSupport.Framework.Core.Utilities.NativeApi.StandardContextTypes

3. node-def (Node Definitions) I believe corresponds to IStructureInfo Interface:
producthelp.sdl.com/.../f80abf83-c92d-2867-a202-2f8d0ff4b619.htm

I believe this just organizes the structure info into a tree describing its behavior. Look at MustUseDisplayName property which explains what you can do with this information.

http://producthelp.sdl.com/SDK/FileTypeSupport/4.0/html/2e87bfce-cdd8-06dd-dcfb-dd64c3f2375d.htm

I'm not an expert though, but hopefully that helps!