Playchoice game information is stored in circuit chips called ROMs (Read Only Memory). There are two types of ROMs used on Playchoice game cards: EPROMs and Mask ROMs. To tell which type of chip your game board uses, know that EPROMs have a small round window on the top of the chip (usually covered by a sticker) that allows it to be erased. Mask ROMs do not.
Example: Super Mario Bros 2 ROMs
Why is this important? Certain Playchoice game ROMs can be swapped among compatible boards. (See also: Customizing Playchoice Game Boards). However, game boards that use Mask ROMs can NOT be interchanged with those that use EPROMs. What's more, Nintendo produced two versions of several playchoice games: one version with Mask ROMs and one version with EPROMs. (See also: Mask ROM Board List) Although the games play the same on both versions, the Mask ROM board is less desirable because it does not accept other games.
It is possible, however, to convert the Mask ROM version of a game to an EPROM version by creating new ROM chips and resoldering certain points on the game board.
See also: Game Board Conversion and Repair