PrF UG slots value type string
From Protege Wiki
The String Value Type
A slot of type String has text strings as values. You can enter ASCII characters for an instance, including upper and lowercase letters, numbers, and the basic symbols on the keyboard, such as !, _, and %. String values can also include spaces.
To set the value type of a slot to String:
Create the slot, or view the Slot Editor for an existing slot.
Select String from the Value Type menu.
Fill in the other fields as desired.