It's easy to get the desired result; the checked items are stored as a return-separated list, so to get them comma-separated, you only need to substitute the return character with a comma:
Substitute ( yourField ; Char(13) ; ", " )
Question is where to put this calculation. In olden times you'd have to create an extra calculation field; nowadays (meaning FM 14), you could add a button bar with only one (1) segment to your layout and use the above as label calculation. Format the segment as desired.
Thank you, this worked.
I ended up using two separate fields, one to choose items in a checkbox. then using a calculation field to display the exclusions for the selected items in a comma separated format