Substantial authority is an objective standard: it asks whether the weight of authorities supporting the position taken — statutes, regulations, judicial decisions, and administrative rulings, among other recognized sources — is substantial in relation to the weight of authorities supporting a contrary position, without regard to whether the taxpayer actually knew of or relied on those authorities when the return was filed.

The standard sits in a middle tier among the different confidence thresholds used across tax practice: it is a meaningfully higher bar than the reasonable basis standard, which merely requires a position not be frivolous, but a lower bar than the more-likely-than-not standard, which requires greater than 50% likelihood the position would be sustained if challenged. Satisfying substantial authority for an item generally protects the taxpayer from the accuracy-related penalty for a substantial understatement attributable to that item, without regard to disclosure.

Because tax positions frequently rest on the practitioner's own probabilistic assessment of how a contested legal question would be resolved, the substantial authority inquiry is inherently a weighing exercise rather than a binary determination. Juricratic represents it as an authority-weighting dial, letting a user simulate accuracy-related penalty exposure as a function of how strong the supporting authority is judged to be relative to the contrary authority.

A tax practitioner evaluating a return position documents the specific authorities relied upon and assesses their weight against any contrary authority before filing, since a well-supported substantial authority determination can eliminate accuracy-related penalty exposure even if the position is ultimately not sustained on the merits.