Mechanistic (biochemical-pathway) classification of inherited metabolic disorders following the consensus International Classification of Inherited Metabolic Disorders (ICIMD; Ferreira CR, Rahman S, Keller M, Zschocke J & ICIMD Advisory Group, J Inherit Metab Dis 2021;44(1):164-177; PMID:33340416, DOI:10.1002/jimd.12348). ICIMD is a three-level hierarchy — 24 categories (layer 1) over 124 disease groups (layer 2) over ~1,450 individual disorders (layer 3). This enum encodes layers 1 and 2 as a single hierarchical permissible-value set: each category is a top-level value, and each group declares its parent category via ``is_a``. Curators should assign the most specific applicable node (usually a group); the parent category is then derivable through ``is_a``. Both category- and group-level values are valid assignments, so an entry may be tagged at category level when the specific group is unknown. Provenance note: the category set and group membership are transcribed from the ICIMD paper's Results section (the 24 numbered categories) and its Figure 1 sunburst (the group ring). The groups here are the primary ("group"-ring) nodes of that sunburst; a handful of the deepest glycosylation and glycerophospholipid subdivisions are folded into their parent group, so this enum carries 113 groups rather than the paper's headline 124. The eight ICIMD "super-domains" (e.g. "Intermediary metabolism: nutrients") sit ABOVE the category level and are recorded in the category descriptions rather than as separate enum nodes.