This feature is being controlled by the Display Type= Show as List that can be only be found if you where to create a custom field with Type= Multiple Select.
I. Create a new Custom Record Type and a custom field with Type= Multiple Select.
1. Navigate to Customization > Lists, Records, & Fields > Record Types > New.
2. Give it a name.
3. Click Save.
4. Click New Field button on Fields tab.
a. Give it a name.
b. Type= Multiple Select.
c. List/Record= Customer.
d. Store Value is checked.
e. On the Display tab, use the given options below:
• Display Type= Show as List.
8. Click Save.
II. Create a new record using the Custom Record Type created in
1. Navigate to Customization > Lists, Records, & Fields > Record Types
2. Click on New Record link.
3. Give this new record a name.
4. Select a few of your customer records on custom field created in
5. Click Save.
Note: If you would have noticed that from the time that you are filling out the custom record form, this custom field with Type= Multiple Select is on the Main body but after you clicked on Save, it has now been moved to the Custom Subtab and it is being displayed as a sublist.