From FreeSpace Wiki
rand-multiple Rand-multiple (Arithmetic operator) Gets a random number. This number can and will change between successive calls to this sexp. Returns a number. Takes 2 or 3 numeric arguments... 1: Low range of random number. 2: High range of random number. 3: (optional) A seed to use when generating numbers. (Setting this to 0 is the same as having no seed at all)
Use this instead of rand if it will be called more than once in a mission. Unlike rand, this SEXP will re-randomize the number for each call.