It depends, some games are rentry only which is like a rebuy except you repay the fee on it as well and unlike rebuying into your same seat with a reentry you redraw seats.
On ftp they have multientry, which would be the situation where you can use them as a reentry or you can run them simultaneously. Basically all entries play seperately unless you have more entries than tables available to sit you at. For example if you have 4 entries and entrants only has enough for 3 tables then your 4th entry will combine its chips with your smallest stack. If the position that the combine tabkes place is ITM then you receive the $$ for that spot, and so on. They also have a thing where if you do get ITM with all entrants and bascially don't bust with any of them so that come the final table all 4 have been combined then you get extra $$$$ like 5k, but IMO near an impossible event to occur and would have to be running like 4xgod lol.