The list function works just fine as long as you want returns between each value and null value items excluded from your list. If you want some other character as the delimiter, use it inside a substitute function call to replace the returns with the text of your choice.
Substitute ( List ( RelatedTable::Field1 ) ; ¶ ; ", " )
produces a list separated by commas instead of returns.
Thanks, guys! ... Dan, for asking the question, and Phil, for providing the simple solution (as he so frequently does). This solved a nut I've been trying to crack off-and-on for more than a year now.
Another data system (not FileMaker) my organization uses had added the capability for users to search its dbase for multiple specific records using a text string of comma-separated ID numbers, but I couldn't figure out how to export those IDs from my FileMaker databases delimiting them with commas. Simply creating a new calculation field that applies the Substitute function to a summary list field Phil had previously helped me create for another purpose was all that was necessary.
Now at the click of a button, I have a list of comma-separated IDs -- no matter whether it's two or two-thousand of them -- copied to my clipboard, ready to paste into the other system's search box.