Lower values indicate more important labels i.e.
If set to -1 or not set at all, then default values are used, depending on the geometry types of the features:
This value does not have to be an integer because it is just a continuum of values for comparison. Users can use this to specify that a class of labels should have priority between two other classes with close priorities.
For example, PreferredRoutes might be given priority 13 to appear ahead of general line feature labels, but after general point feature labels. And HistoricSites might be given priority 12.5 to appear ahead of PreferredRoute labels, but still after general point feature labels.