Themes allow customisation of AAF by tuning it to ignore some installed animations. If you do not install some themes Theme, there will be no tags added to any relevant animations you have installed so they should not be picked by mods using the tagging system, So if you want to install an animation pack that has a mix of Posing and Other animations, you should only experience the Posing ones in your game unless you manually pick them in the AAF wizard.
You should only see the message when a theme is first installed on a save game.
https://bitbucket.org/dagobaking/advanced-animation-framework/wiki/Guides/WhatIsAnAAFTheme

This includes avatars for all the creatures made by Poliistiro and used with permission

And you can safely install multiple Themes but if there is any overwriting the last theme installed will win of course.
If anyone has any extras to add send them to me and I can add them or its possible there are some errors to correct. I'm pretty active on the AAF Discord.

These theme packs contain all the parts of tagData from Halstroms_TagData.XML (APT) split into each relevant theme making APT redundant. If you have APT (Halstrom_tagData.xml) also installed it will result in double tagging some animations and probably cause weirdness or bugs.
This also includes new avatars for Humans and most creatures by Polistro.

What happens is currently these themes add more data to animations so they can be sorted better a tag line looks like this:

<tag position="F-Dance01-01HipShakeStart" tags="F,NoFurn,Dancing,Neutral,Held0,Love0,Stim1,Dom0,NullToSelf,SavageCabbage"/>

The position is the tags name or ID and if the AAF system detects it the tags will be checked for any animations. The "F" tells any mods its a single actor animation, Female only. NoFurn means there's no table, bed or couch required to do this animation, it can happen anywhere.
Dancing means its a Dancing animation. Neutral Means its neither Loving or Aggressive, Held0 means the actor is not restrained. Love means it does not increase or decrease the love between actors involved, Stim is the amount of pleasure it causes, Dom rates whether the receiver gains either submission or Dominance from the animation. NulToSelf means there is no direct stimulation from the receiver to herself. SavageCabbage is the Animation Author.

It currently supports tagging for the following Animation packs:
Real Animated Poses: https://www.nexusmods.com/fallout4/mods/31455?tab=description

Thanks to:
The F4SE Dev guys,
The MCM Dev guys,
Dagobaking, CG! and other AAF testers
Polistiro for the Avatars included in this Theme