This property type is used to choose one item from a list of valid values supplied as the right argument to the type name in the class specification. For example:

Compression : OneOf 'None' 'Low' 'High'  © Algorithm

You may give a default value, which should be one of the valid entries ...

Compression='Low' : OneOf 'None' 'Low' 'High'  © Algorithm

... and you may hold the list as a variable in the class namespace ...

Compression : OneOf ‘options © Algorithm

Note that the expression giving the default value is executed by Class.ParseSpec in the class namespace when you run CPro.Build. The expression giving the option list is executed by the designer whenever you switch focus to an object of that type, causing the property-watcher to rebuild itself.