In the settings for Skin List Label 25 (and probably earlier) under the Symbol Label Custom Settings and in Properties Content...
There are check boxes for:
- Show "undefined" Values
- Show Unavailable Values
What is an Unavailable Value?
Also there is a data entry box for --> Sign of Unavailable Values
How, Why, Where is this used?
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Undefined properties are properties in which the user has not yet entered a value or default
Unavailable properties are properties that are not applicable to that element as in a property that has a different classification then that of the element's classification.
The sign of a unavailable property is just a user supplied value that will show up in the value field of the undefined property rather than just a blank. This gives user feedback that the property is undefined and not just empty.