We don't have synonyms for substyle yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are subtle, subtile and substile.
(n) :
A right line on which the style, or gnomon, of a sundial is erected, being the common section of the face of the dial and a plane perpendicular to it passing through the style.
(n) :
A right line on which the style, or gnomon, of a sundial is erected, being the common section of the face of the dial and a plane perpendicular to it passing through the style.