Backing up OS 10.15 to external USB disk what format should the disk be?

I use ChronoSync (besides from TM) to back up my OS to an external USB disk and to a network drive.  Presently I am using 10.13 but will soon move to 10.15.

In order to back up properly I like to know what format the USB disks should have to make sure that if I need the backup it works perfectly.  Or if I want to use one of the USB backup disks to start the computer up from, like in an emergency making sure that it works flawlessly.

Thanks.