On forms, the reference lookup icon (Core UI reference lookup icon) appears by editable reference fields. Clicking the reference lookup icon displays a list of records on the referenced table.

When List v3 is enabled, the list appears in a popover. Otherwise, the list appears in a pop-up window.

Figure 1. Reference lookup list popover
The reference lookup list opens in a popover when List v3 is enabled
Figure 2. Reference lookup list pop-up
The reference lookup list opens in a pop-up window