[alphonsegaston]

Sure, but formal rules are not equivalent to a prescriptive grammar. AAVE has formal, consistent rules, described by linguists. You can make mistakes in AAVE just as in standard English (see: "African American Vernacular English Is Not Standard English with Mistakes" https://web.stanford.edu/~zwicky/aave-is-not-se-with-mistake... )

like most languages in history, AAVE has not gone through a process of codification and standardization to formalize it

I'm not sure what you're getting at - prescriptive grammars of the codified form you're describing are the products of their political and economic circumstances. There isn't some Hegelian trajectory of linguistic validity, where all variants aspire towards legalism.

[morgante]

If not the process of standardization, what differentiates formal and informal rules?

[alphonsegaston]

Formality, if present, can be equally derived from observation and description of usage, using the linguistic analysis of the kind found in the articles I linked above. Language, when viewed in total, has shared elements and patterns that supersede the narrow focus of prescriptive impositions. It's why linguists can observe things such as that the dropped copula exists in both AAVE and Russian.