s are those occasional crosswords you see which have the gimmick
of having to put a special symbol in a square which represents a word or other combination of two or more letters in the words passing through that square.
In these puzzles, you'll either have the same symbol used several times throughout the grid, or you'll have a set of related symbols that may be used once each, or repeated.
For instance, you could have to put * in certain squares to represent "STAR" in the crossing words, or you could have to put a color in certain squares, where various colors are used throughout the grid.
In a rarer variant, the symbol may be interpreted two different ways in the different directions. For instance, you could have 1984 (the book) as an entry, and have the 1 represent the ONE in THRONE in the crossing word, 8 represent EIGHT in WEIGHTLIFTING, etc.