Hello, I'm trying to figure out how frigate upgrades work. I know they level up by sending them on missions, but does it matter what type of mission?

Say I have 5 free frigates. 3 trade frigates and two combat frigates. If there's only a Trade mission available. The trade frigates alone is at a 3 star level and the mission is at a 2 star difficulty.
Should I send all five ships to this mission or only the trade ships? If I send all 5, will the combat ships still level up their combat stats or will they level up their trade stats?

Feels unnescessary not to have all frigates out at missions all the time, but don't want to "spend" upgrade points on the wrong characteristics.. :-/

You must send the right ships for the mission depending what sort of misison you have, for balanced missions add a trade, an industrial and a combat ship for example.

How many ships should you add to a mission? You must have your 'Fleet Rating' higher than the mission difficulty which both are expressed in star numbers, for that you need to add just the right number of ships, not more. If you have a FR with 2 stars more than the MD, then you will hardly ever need to repair your frigates.

The frigate upgrades will increase a specific area of the mission like trading, exploratrion or combat if you install them.
Support frigates can lower the fuel requirements for a mission if you add such a ship, but that's not always the case, look if it does and if not remove the frigate from the mission list.

As you send out missions all frigates involved will get better over time increasing in Class for even better performance. Better performance means you need less of good ships in a mission to increase the Fleet Rating and often if you send a S-Class ship together with a S-Class Support ship you won't even consume fuel for the mission.

These are the basics only :)

First of all, thanks for the response!

I might have been a bit unclear. With upgrade I'm talking about the stats that increases along with the frigate level ups.
It sounds like, in my example, it would just be a waste of time (or rather a waste of fuel) to add combat frigates to a trading mission (?) My thought was that it was better sending them along than not sending them at all. Just as long as the fleet star ranking was higher than the mission required ranking.

I want my combat frigates to get as high combat ranking as possible. Same goes for the trade frigates, explorers and industrial. Not sending a frigate on a mission wont level up the frigates - so my main question is if sending the wrong type of frigate to a mission still would help it increase its main stats or not :)

The stats you are talking about - and that you can see separately in the window - will sum up to evaluate the Fleet Rating. My tip is you should have all separate stats for the mission at least on number 10, but what counts is your Fleet Rating in stars. Try out different ships and see how your FR will change and how much fuel the mission consumes. Keep it easy.

Okey, but I'm not worried about the missions, only the level ups of the frigates.
To make another hypothetical example:

If I would send a combat frigate only to trade missions. Would the frigate still get better combat skills as if it were sent to only combat missions? Or would it instead get higher trade stats? (don't way in the possibility for the combat frigate to finish the trade mission or not)

The frigates will gain 'experience' no matter what type of mission you send them on. When they level up, they gain 6 stat points. Each of the traits (combat, exploration, etc.) increase by 1 point with 2 other points randomly distributed among the four traits. The points are not distributes in relation to the class, type or anything else to do with the frigate. Their growth is related solely to the seed of the frigate and cannot be changed through reloading etc.

So, bottom line is send those frigates out and they'll gain experience. Whether or not they'll max their combat stat is dependent on their seed, not the mission type they're sent on.

Also note that frigates stop leveling up after 55 missions. Although the entire article is helpful, scroll down to near the bottom in the Rank-Up sections: https://nomanssky.gamepedia.com/Frigate

Great, thanks for the tip!