2.1.1869 Part 4 Section 184.108.40.206, sub (Subscript (Pre-Sub-Superscript))
For additional notes that apply to this portion of the standard, please see the notes for oMath, §220.127.116.11(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.