Every slot has it's own limitations and rules.
For example, if all your cards are level 1, it's best to stick with a slot that has a novice level cap. If you don't have a lot of gold cards, don't take the slot where you are only allowed to use gold cards, etc.
Oh wow, where can i see that? I use brave browser on my mobile to play it. Can i see the rules for each slot in mobile?