While creating retained earnings ob53, system requests to specify chart of account. There can be several co. Codes assigned to a single chart of accounts. FS00 have co. Code segment, that means specific to a co. Code. While FSP0 is chart of account level.
So if there is only one co. Code associated with the CoA, then fsp0 or fs00 will be fine. However if there are more than one co. Code assigned to a CoA, then create a retained earning account with fsp0 which is CoA level.