Actually the problem is too many people here are experts that use acronyms without explaining what they are like as if it's part of laymen's English now.
For example: NEXT. What the hell is "Next"? Is this a version of the Librepilot GCS?
Is it a GPS version? What is it?
In the attachment you have screen shot of searching forum with "next", "what is next" terms. Second hit in both cases.
It is a git branch of that name, which holds development code. Please don't ask that again in another thread.
Another Acronym I dont get is I2C. Why not just call it AuxMAG? What does it mean I2C? What does the I and the C stand for?
You asked the same thing in another thread.
What I2C is, please refer to the
first hit
http://lmgtfy.com/?q=I2C it will link you to good overview what I2C really is, but it is communication protocol between two devices.
Why it is not called AuxMag is simply because AuxMag is the device itself, and the same device can communicate with flight controller in different languages (protocols).
Finally: What's the difference between Complimentary(Basic) and INS13Indoor?
Please have a look at Wiki
https://librepilot.atlassian.net/wiki/display/LPDOC/INS13+-+GPS+NavigationBy applying simple logic, INS31Indoor is the INS31 variant that you can use indoors. So it has all what INS31 has, except for sensors that cannot be used indoors. What those sensors can be ? Hint "clear sky is needed".
Complementary is a simple sensor fusion algorithm designed for low end micro-controllers back in AVR 8bit era. They couldn't handle full statistical model such as EKF.
There are whole lengthy books about it and people study. For the purpose of this post I think good simplification would be to think of EKF as properly sound mathematical model, while Complementary you could consider like approximation (hack).
Pros and cons are describe in the above Wiki link I provided.
People put a lot of effort in making this Wiki as complete as possible. Please if you don't know something search it for the terms you are confused. It is likely that your question has already been answered there.