myconnectindex is a gamevar which hold the ID of the player using the local computer in a multiplayer game. In single-player it's always zero.
It is of little actual use in con (despite the impressions of some mod authors who misunderstand dukes netplay). In singleplayer it will always be fixed at the first player index. In multiplayer actor code it will cause different results on different computers which will almost certainly cause some form of desync. In display code screenpeek should be used instead to allow viewing of others screens in coop.
Initially player indexes go from 0 to numplayers-1 however they do not change when a player quits so you will end up with non continuous indexes.