PDF 1.7 defines both ‘Figure’ and ‘Formula’ Structure Element types for illustrations (as well as ‘Formula’ for interactive forms).

AH Formula supports MathML 3, and it automatically tags an fo:external-graphic or fo:instream-foreign-object that contais MathML as ‘Formula’ and tags every other fo:external-graphic or fo:instream-foreign-object as ‘Figure’. It is not known what other formatters generate for MathML.

If you have to use a graphic, and not use MathML, to represent mathematics, then use ‘Formula’ to identify the graphic as a formula. You should also use axf:alttext (or equivalents) to provide alternate text for the image.