This field will show a dropdown menu with all the records that exist within the selected App. For this field type to work, you will need to, first, create a Record Reference field that has as the Reference Object 'Apps'. This is necessary to know which records to display.
You can find a use case example for this field in the Tasks Apps. Since you can create Tasks for any record for all your Apps, you first select the App and then the specific record.
You could create Record Reference for a Resource App, where you can track research results and tie them to specific records regardless of the App.
Configuring a Record Reference Field
Give your field a name and add a field hint if desired. You can then select the section in which the field will be created or keep the default option.
The field type must be set to 'Record Reference'.
Then you will need to select the Reference field you created for the 'Apps' Object in the App Reference field. Here we are telling the system to show us a list of records from the App we select in the Reference field.
Enable the 'Required' checkbox if the information for this field is mandatory. It means that if you enable this option, every time a team member creates a new record, they will need to select a record in this field in order to save it.
If you enable the 'Lock After Create' as its name implies, the field will be locked once the new record is saved and the value of the field will not be editable afterward.
Once you are done setting your field, click the save button.
Note: All new fields are added at the bottom of the form.