If a pawn or lance is moved to the furthest rank, or a knight is moved to either of the two furthest ranks, that piece "must" promote (otherwise, it would have no legal move on subsequent turns). A silver general is never required to promote, and it is often advantageous to keep a silver general unpromoted. (It is easier, for example, to extract an unpromoted silver from behind enemy lines; whereas a promoted silver, with only one line of retreat, can be easily blocked.) A rook, bishop and a pawn is almost always promoted, unless there is a problem due to "mate with a dropped pawn".