Data of the Enum, or Enumerated, type is data with a fixed, limited number of options. Usually (as far as to be seen in Titan) the options need to be given as String.
As far as known, the possbible values for a property of type Enum should be denoted in the property's wiki post - see the properties below in Also used in
Examples:
Some (few) enumerations are listed in the online documentation - just scroll to the end in the left-hand contents frame, e.g.