This flag is given to all things in MP that were created on another player's computer. All things originally in the level are managed by the server, so they do not have the 0x100 flag on the server's computer - on every other computer they do.
Walkplayers are exceptions, they must have 0x100 on every computer except the computer of the player that's using it. So there is only one walkplayer without the 0x100 flag on each computer.
It is unclear why the 0x100 flag is used with things of all types. Weapons, for example, cannot be damaged or killed, but the flag is given to them anyway. This is perhaps done as a safeguard or to show which computer created the thing. This flag may be more symbolic than realized...