This field, as its name implies, allows you to reference specific information of a given record from other Apps. It means that you will be able to get information that already exists in other Apps to avoid having to enter it twice and also to keep it consistent between your Apps.
It is mostly used to reference, for example, information about the organization in the Opportunities App, to avoid having to enter it twice.
To create this field type you will need to specify the App you want to get the information from and then the specific field where the information is stored.
Configuring a Reference Join 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 'Reference Join'.
Then you will need to specify the App you will be referencing under Reference Object. This field will include all your existing Apps.
Once you have selected the App, you will need to choose the field.
One common example is to make a reference for the address from the Account in order to add it to the Opportunity's detail as well. This will save time for your team when creating Opportunities. For this example, the Reference field will be Accounts and the field would be 'address' or 'location'.
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.