I've given my definition.
If every gene in a snake can be found in the wild population for a snake, then it is pure.
If a rat snake introduces its genes to the wild florida king population, those genes are now part of the wild florida king population.
There is no way to catalogue every single gene in the wild population of a subspecies, so in captivity, if you care about purity you do so by not hybridizing with other species or subspecies. If you don't care, that's your business - integrade away, a lot of really cool looking snakes are the results of designer integration. But please be honest about it.
If I cross a cal king from one locale with a cal king from another locale, I can not call the offspring locale specific. Why then should I be able to call a snake a brooksi that has non brooksi genes in it? I shouldn't - it'd not a brooksi, it may be a gorgeous snake - but it is an integrade or a hybrid.
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3.0 WC; 0.1 CB L. getula californiae
0.1 CB L. pyromelana pyromelana
0.1 WC; 10 eggs (7/11) Elgaria multicarinata multicarinata