However, the field is primarily for SDK applications and is not available for editing in and you cannot have XML string fields on New Record forms.

Additional information on the SDK can be found:
SDK Documents - See Creating Lookup Sets.

Once you this format is selected, the following options become available:

Insert Lookup Items from the following Lookup Set - select a Lookup Set for this field.
Lookup Sets are pre-defined sets of data/lists from which items can be attached to objects.
Default Value - set a default value that will appear in the field when it is attached to an object

See also Creating Additional Fields for definitions of the other common fields, for example, Name and Abbreviation, that appear on this dialogue.

XML string Additional Fields