Source: TW
Jurisprudence
Dharmasastra-s were not like legal codes, but works of jurisprudence and legal philosophy, which reflected and framed achara-s themselves in the light of Vedic philosophy.
Primary daily practice and enforcement of law and norms was at the hands of achara, which were decided by consensus or legislation by the community or region’s wise men and elite (with the tacit support of the people). Achara-s themselves did not follow according to Smrti-s as a hard rule, rather the “introspection of acharas” (dharmasastra) was used for jurisprudence, legal reasoning and framing how to do legislation.
Codes
Smrti-Bhasya-s and Nibandha-s of those were used as the primary source for a group if they wanted to amend their achara-s to add or modify something, additionally also modifying sastric precepts from the sources for their local communities.
Generally, you had achara-s that were of non-Smrti origins, and those that were adapted or modified from commentaries and digests that interpreted Sastra.For the former, Mimamsaka-s invented the doctrine that any achara that cannot directly be derived from an existing Sastra must come from a lost one. It’s obvious this was a clever workaround of the rule that law must rooted in smrti-sruti to allow for some freedom on the ground level.