What are the precise requirements for Plus codes

[Nikato Muirhead]

I understand that a  full plus code is 10 characters and a + sign, so 11 characters altogether.Is it  safe to say that 10 sets of characters A-Z 0-9, and a plus sign  are enough to produce  any possible plus code ?   Are there letters  that are forbidden from being used in a plus code ? 

[Andreas B]

The full alphabet for a plus code is "23456789CFGHJMPQRVWX".

In addition to the 10-digit variant, other lengths are valid as well - all even lengths in range [2..10], all lengths in range [11..15]. For lengths shorter than 8, "0" is used as padding character (e.g. "CFGHJM00+").

The first two digits do not use all 20 valid characters, but only the first 9 or first 18, respectively (e.g. "XX222222+" is not a valid plus code).

All of this can be found in https://github.com/google/open-location-code/blob/master/docs/specification.md