Matching is a exact match, including case. What is matched is only what is displayed. You get no access to underlying HTML code in the body of the message.

- Does [Contains] match exact string? -- IE "special offers" as a whole

If that is what you put in as the criteria, including spaces.

- Does [Contains] match each word in string? -- IE "special" "offers" as two words

No

- Is word to be matched case-sensitive? -- IE "SPECIAL" is same as "special"

No.

Also, if there is an in-depth documentation somewhere, please give me a link.

There is not much in depth to go into. The source code is here https://searchfox.org/comm-central/source/mailnews/search/src/nsMsgFilter.cpp that is about as indepth as it goes.

Personally I like the addon filtaquilla as it offers regex and javascript extensions to the base among of host of other features. https://addons.thunderbird.net/en-us/thunderbird/addon/filtaquilla/

Mozillazine has an oldie but a goodie here https://kb.mozillazine.org/Message_Filters

I've found the documentation and that was no more help than how to create a filter.

Is that this https://support.mozilla.org/en-US/kb/organize-your-messages-using-filters

I strongly suggest you have a look at the contents of the msgfiltersrules.dat file in your profile folder, it is a text file and how you filter is laided out there is actually rather instructive as it includes the implied OR's and AND's from the dialog match any and all options.

Yeah.... Shortly after, I found FiltaQuilla. By case-sensitive, I meant insensitive. Total typo. So far, filtaquilla is doing exactly as I want while being more accurate. Other base features are nice, just needed better filters. Now if only I could filter by gmail group. With gmail, you can have folders on there and I tried to get most of the filter work to be done by Google and the deletion filters done locally but that does not work unfortunately... It's either email or local folder.