Hi Ynez

Is 'S' a variable letter (indicating the drug substance, perhaps), or is it
an alphabetic sequence number (A, B ...)  or is it always S? In the last two
cases, this structure can be achieved by setting up numbering for Headings
1 - 5 following the instructions in

http://www.shaunakelly.com/word/numbering/OutlineNumbering.html

If however 'S' is a variable letter specific to the drug, then the problem
is how to set the letter. It would be possible to do this by using Format >
Bullets and Numbering at the 3.2.S level (Heading 3), to change the 'start'
letter at that point. But this approach is very limited (and dangerous)  if
you need to copy and paste material between sections. Unless you are very
clued in to exactly how numbering and lists work, I think this is almost
bound to end in tears!