Create list of record IDs selected from a Lookup Multi-Select field

Create list of record IDs selected from a Lookup Multi-Select field

What field type would hold a list of the ID numbers of records selected from a Lookup Multi-Select field, and what does the script look like to create that list upon user input at that Lookup Multi-Select field?  As always, I super appreciate the help offered here.

Practically speaking, what I want to do is to reset records that were updated via the Multi-Select field when the form was first created, that in Edit mode have been de-selected.