As you may now users can search in list columns and search form fields with multiple search terms. For example, in Vendor column: 101;202;303. This finds any Vendor record with 101 or 202 or 303.
When using a select list to find your values of 101;202;303 you may have to use multiple searches or multiple pages of 10 or 20 records in order to even find your search values. This is one constraint today. Another constraint is re-launching the select to find additional values, today they are not brought forward into the select.
Below are our working POC ideas to solve these constraints:
Here a user selected eight Manufacturer records that they eventually want to search for:

Here we are showing how previously entered or selected multiple values from the main list are retained in the select list for users to continue to refine:

Here we are showing many many records selected and how overflow (scrollbar) will look:

Users can either click the X to remove the record or uncheck the item from within the list grid itself.
We would love to hear your thoughts on these designs and if we missed any use-case you are currently struggling with in this area.
Thanks!