Except as a preposition is used to express a qualifying limitation to a rule or statement.
In its prepositional form, except is used to introduce something that isn't included in a previous statement, which otherwise could be assumed to be included in the scope of the statement. These could be people, objects, or concepts.
1. We invited everyone except for Joe.
2. She read books except for the novels.
3. We played every game except for the card game.
It is important to remember that when except is used as a preposition, it only suggests that whatever follows is excluded from a previously stated context. There may be other elements or exceptions to the statement that are not mentioned.