it's base 24. after going from single character alpha to omega, then you introduce the second character: alpha alpha, alpha beta, ...
for variants it could be minor versions such as sars-cov-2.4 but then there may be confusion about them being related sequentially and that one is an "improvement" over the other. :)
[1] https://www.mathsisfun.com/numbers/cardinal-ordinal-nominal....