Right now there is no way to control the order of map series groups. It appears to be alphabetical ascending with no other options. I had previously set up my group field's domain so that the codes went in the desired order, but could not get the descriptions to appear in the TOC tree ( Map Series - use domain descriptions as group name... - Esri Community ).
Now that I made it a text field, they're in an order that I don't want. I don't want them alphabetical.
