BI RespawnTemplates command lets players choose weapon loadouts and/or a spawn position via dialog after they die. RespawnOnStart is a command that works hand & hand with the respawnTemplates. respawnOnStart is supposed to bring up the respawn dialog upon mission start, so players can initially choose their loadout and/or spawn position.
There are work-arounds though. Such as killing the player(s) at the start of the mission to trigger the respawn dialog. Or simply denying players the ability to initially choose their loadout and/or spawn position.
Both of those work-arounds really stink though. If respawnOnStart weren't broken, using respawntemplates would be a viable option for missions. Mission designers would have a simple way to implement a nice, fully functional respawning dialog system into their missions. And without some halfassed workaround or creating their own dialogs from scratch...