Every language/project has their guidelines:
Perl likes two digits after the period. Less than 1.0 means less than perfect.
Some projects do even/odd numbering in some component of number to mean stable/devel (respectively).
So I'm using major.[minor][subminor], where major is still 0, not being perfect yet. It's approaching 1.0, though. Then the next major is generally for backwards-incompatible changes, which hopefully never happens with a backup tools, so a 2.00 will probably never appear. Until maybe it's a rewrite and plugins need to be rewritten, but on-disk format would still be compatible.
Some people's version number is just the date. Or some are just a sequence of release numbers, starting with 1, then 2, then 3. Like GNU less
, which is apparently at 382 now.