i just found this topic because i was having trouble with Section Tag Types not transfering any Section Types (system family) over to another project with the project transfer function.
What got the job done was simply copy pasting existing sections from one project to another.
1. Copy a section (wich is set to the section type you wish to transfer to another project) on floorplan to clipboard
2. Paste the copied section on a floorplan in the other project
Result:
the section gets pasted and the section type gets added to the project.
You can delete the section after pasting it or modify it as you wish.
Side note:
Really hope they will add the option to transfer section types to the project transfer tool,
i thought that it used to work like that before , before section types got renamed to section type tags