Kintyred
Executive Stream and Main Stream differentiate all officers at or above wg cdr (or is at sqn ldr?). I can't quite remember the full details but it is basically have you done ASC or not? If yes you're exec stream, will get shorter (2-ish year tours), be expected to move around more regularly in the hope that greater things will be coming your way. If you are a main stream wg cdr, you can expect (but not be guaranteed) longer tours giving stability for family and in your job.
Posts are supposed to be annotated ES or MS, so expect the nice sqn/wg command tours to all be ES and the (how should I say this?) duller staff tours that aren't need for the career thrusters be left to MS officers. Of course, the exigencies of the Service will over ride all of the above as required.