When creating a board, or in the board settings screen, you can choose which weapon will be tracked by that board. You can select foil, epee, sabre or mixed.

If your board is mixed then any tournaments you create will also prompt you to select a weapon, otherwise tournaments will default to match the weapon of the board.

There are only minor differences between the different board types for now, unless you are seeding by, or exporting USFA member tournament results in which case it will be used to determine the appropriate rating for your entrants.