2.1.1869 Part 4 Section 126.96.36.199, sub (Subscript (Pre-Sub-Superscript))
For additional notes that apply to this portion of the standard, please see the notes for oMath, §188.8.131.52(d).
a. The standard says sub only specifies the subscript of sPre even though the schema also allows it as a child of sSub, sSubSup, and nary.
In Word, sub specifies the subscript of sPre, sSub, and sSubSup and specifies the lower limit of nary.