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mvhulten Third Party means its from someone else. So the License could be anything.
Audititing could be , but most of the time the is no auditing
The Reason not Putting in the Offical are diffent , mayby some examples
1. For Developing Addon, and testing
Since a submission to the Offcial Repo Could take some time
https://github.com/xbmc/repo-scripts/pull/691 (Waiting since December for Submission), so third Party Repos are a great Way for Testing
2) You don't want to follow some rules.
For example you have written an "Dutch Addon", that only is for Dutch People. You have to Put all "Text" in a Textfile and have to translate it in English and Dutch. If you dont want to make this efford, you must use a third Party Repo
3) You Dont like some behavor of people from kodi.tv , so you don't want to work with the offical
4) You write an addon for not Supported Kodi Versions, you cant put them in Kodi.tv anymore
5. You Use Transparent Pictures,or somethin isnt conform to other Rules
6) You Want to Track how much an addon is downloaded
7) you write an addon you want money for
8) You want to archive something on your Own
9) The Adodn is Legal, but Uses Parts of an Illegal Addon
10) You extend an Offical Addon with functions, the Owner don't want. So you can Fork it, bit i'm not sure if it would be a good Idea to have 2 time nearly the same addon in the Offical (it the nearly the same, specially when the Original Developer don't like it)
So as you can see the Reasons are quite different. But beware. An Third Party Addon/Repo Could Harm your System, so you should know if the Repo ist trustworthy. The Best Idea is to make your Own Picture of the Maintainer of the Repo