Like Trello, users who have access to the board can see all cards on the board. It's not possible to show only a subset of the cards to a specific user, unless you create a separate board for this user and then mirror the cards from your source board to this user board.
For example when you have a master board where you track all of your work from different customers, you can create client boards that contain the work for the specific clients. Then when a client has access to the client board, the client can only see the related client cards.
Adding a member to a board will grant access to all cards. Even if you filter some cards, the member can remove the filter and see everything. It’s also not possible to “not import” all cards from Trello to Placker.
An alternative in this case is to, instead of adding the person to the board, create a separate board and mirror only the non-confidential cards to that board.
If you would like to ask questions about this method, feel free to