The $procedure-step-numeration parameter controls the numeration style of nested steps and substeps in a procedure. Steps at the top level will use the numeration style of the first character in $procedure-step-numeration, substeps at the second-level will use the numeration style of the second character in the string, etc. If the step depth exceeds the number of characters in the $procedure-step-numeration, selection “wraps back around” to the first character.

In other words, if the string is “1ai”: steps at the first, fourth, seventh, tenth, etc. depth will have arabic numeration. Steps at the second, fifth, eight, eleventh, etc. depth will have lowercase alpha numeration, etc.