there is 3 slots for your saves to be when playing rejuv, the 'active' one will be the one it shows on load up and the other two can be considered 'dormant', while your backups are just previous places you saved at.
to set your saves to various slots it's all in the name:
Game.rxdata (the .rxdata is just the normal extension of it, the file name would just be Game) would be your Save File in Slot 1
Game_2.rxdata would be Save File in Slot 2
Game_3.rxdata as above
you can switch between these 3 set files freely by choosing 'other save files' when rejuv is loaded up and showing your current team and stuff.
renaming a backup to any of those 3 names would make it in those slots