The "List of" summary field has to be defined in the table where the field you want to "list" is defined so you could define this field in the Items table, but then place it on the Set layout.
But you can also define a calculation field in set that uses the LIST function to list your items: List ( Items::_fkSetID ) or whatever field in Set that you want to use to produce your list.
A third option is ExecuteSQL, which can produce such a list, sort it, filter out values from records that don't meet specified criteira, omit duplicates...
Is there a way to create another field that will not show the result in a (delimited) list but as a text string? I tried to use the getastext, and trim functions but they did not work. Ultimately, what I am trying to get in my example above is a field result that shows the list as a line of text as follows "1 22" (with a space between the first ID and the second ID).
All of the options that I mentioned produce a text string result--but with returns instead of spaces between the values.
ExecuteSQL can specify the space character as the "record delimitter" to produce what you want.
Substitute ( List ( Items::Field ) ; ¶ ; " " )
Substitute ( Items::ListOfSummaryFieldHere ; ¶ ; " " )
would also produce such a list of values.