Took me a while when I started out with FrameMaker, but then the penny dropped: you can run more than one counter in the same sequence. Consider the following example ...
1. numbered - tag :listStart
2. numbered - tag :listNumber
3. numbered - tag :listNumber
a. lettered - tag :listLetter
b. lettered - tag :listLetter
c. lettered - tag :listLetter
4. numbered - tag :listNumber
5. numbered - tag :listNumber
a. lettered - tag :listLetter
b. lettered - tag :listLetter
6. numbered - tag :listNumber
The three styles all use a sequence identified as L:
The numbering set-up is as follows; note that I've replaced the standard angle brackets with braces, in case anyone's overzealous browser tries to treat counter fields as mark-up tags.
:listStart resets the numeric counter to 1 and clears the alpha counter
L:{n=1}{ =0}.\t
:listNumber increments the numeric counter and clears the alpha counter
L:{n+}{ =0}.\t
:listLetter skips the numeric counter and increments the alpha counter
L:{ }{a+}.\t
FrameMaker numbering is a thing of beauty!