FCUseCache[func,{arg1,...},{opt1...}] evaluates func[arg1,...,opt1,...] and caches the result such that the next evaluation of same expressions occurs almost immediately. This caching also takes into account DownValues and global variables that enter into evaluation of func.
For example, ExpandScalarProduct can’t be naively cached, because its result depends on the DownValues of Pair and ScalarProduct, which may be changed multiple times during the session by setting and erasing values of scalar products. With FCUseCache, however, caching will work properly, as FCUseCache knows the dependence on ExpandScalarProduct on those DownValues. For all this to work, a function should be explicitly white-listed in FCUseCache.