The $orderedlist-item-numeration parameter controls the numeration style of nested orderedlist elements. Items in the top-level list will use the numeration style of the first character in $orderedlist-item-numeration, items in the second-level list will use the numeration style of the second character in the string, etc. If the list depth exceeds the number of characters in the $orderedlist-item-numeration, selection “wraps back around” to the first character.

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