I hear people say "I can get this or that out of these skins" but how do you know what you can get out of a skin?
The comment from GiantTree is correct, however I am answering here so the question can be closed and to give a more specific answer. Trade up Contracts give out skins based on what skins you put in. The output skin is both one grade higher than the inputs (Industrial will give you a Mil-Spec, Mil-Spec will give you Restricted, etc) and must be from one of the collections of the skins input.
For example, you could put in 5 Glock 18 Catacomb skins and 5 AK-47 Elite Build skins. Both of those skins are Industrial Grade skins, Catacombs is from the Chroma collection and Elite Build is from the Chroma 2 collection. Thus, you can get out of it any Mil-Spec skin from either Chroma or Chroma 2 (namely, Dual Berettas Urban Shock, Desert Eagle Naga, MAC-10 Malachite, Sawed-Off Serenity, AWP Worm God, MAG-7 Heat, CZ75-Auto Pole Position, UMP-45 Grand Prix).
Most of this information is available on the wiki plus additional information regarding exclusions and exceptions.
The whole idea of trade-ups is pretty simple. It provides an alternative to simply opening cases or trading weapons with other players. In order to use the trade-up contracts, all you need is 10 weapons of the same grade.
The grades go in the following order:
Consumer Grade > Industrial grade > Mil-spec > Restricted > Classified > Covert
How it works?
Essentially, if you were to use 10 weapons of the Restricted quality, you will get 1 weapon of the Classified quality. If you were to use 10 weapons of Classified quality, you will get a single Covert weapon. Etc.
Here is an example:
Scrolling over an item in your inventory reveals a list. Something like this:
This is called a "Collection". Basically, if I were to trade up 10 M4A1-S| Atomic Alloys, I would have a 50-50 chance of getting either a AK-47| Vulcan or a M4A4| Desert-Strike.
What about weapons of different collections?
Simple. Having more weapons of different collections in the same contract means that there are more different variations of weapons if you get. If I were to throw in 5 M4A1-S| Atomic Alloys and another 5 M4A4| Dragon Kings:
I have a chance of getting the following weapons:
- AK-47| Vulcan
- M4A4| Desert-Strike
- AWP| Man-o'-War
- Galil AR| Chatterbox
Basically all you need to remember is that when you place weapons of let's say Collection A into a contract, you will get 1 of any weapon that belongs in Collection A. Just that it will be 1 grade "Better".
- Knives, which have the grade Exceedingly Rare ★, CANNOT be used in contracts
- You can now use Stat-Trak weapons in Trade-ups
- I am not entirely sure, but I believe that "Wild-Card" skins, are essentially that one or two unwanted skins that have been placed in the contract just so as to fill it up. Since there is only 1 or 2 of them there is a low chance of their collection being picked, but still, it is possible. Hence they are "affectionately" referred to as "Wild-Card" skins.
I made a little script which helps you to find a safe tradeup. Its already not perfect but it gives you a advice.