[dnanian]

Since the source is APFS, and there are features of APFS that cannot be replicated on HFS+, such as sparse files, it's wise to use the same format. It's not required, but, for example, a 2TB "sparse file" on the source that might only have 200K of data in it will take up 200K on the source and 2TB on the backup...and will be 2TB once restored, if you use HFS+.