2.1 But I want to number some of them…

This is unnumbered in all formats
Definitions 2.3:
This should be definitions 2.3 in all formats

2.1.1 Out of interest…

What happens if I try to number an environment I have specifically declared to be unnumbered? e.g. using {newtheorem, env=‘Examples’, label=“argh”}

This will fail to compile for PDF and it will produce “Examples (#Examples:argh)” as the title in other formats.

The only real benefit to be gained from specifically declaring an environment to be unnumbered is that you are less likely to accidentally number an instance of it!