The deputy feature is always valid for one user/player within one team. That means, if the user is in several teams and the substitution should be valid for all teams, you have to repeat the registration process in all teams.
The coach has no possibility to "determine" deputies for his players. This means that only the user himself can determine and invite the substitutes.
Deputies are not visible for the rest of the team. So for the rest of the team it always looks like the user himself is acting. There is one exception: In the chat, it is indicated whether the user or a deputy has written a message.
The administration and purchase of premium memberships is not possible for deputies. Only the user is allowed to do this. However, the user has the possibility to make a deputy the "new" "main user" and to transfer the corresponding premium functionalities to him.