So when you load a featureset then all features are available to you to view and process. Those are the features that I would call the workable features. With a definition query it is exactly the same thing but now you have filtered the available features via a query. Its like you have taken a featureset, selected your features, and then created a new featureset. Except all that's done in memory with a parameter like .DefQuery or .VisibilityExpression.
~AGP
Dim fs As IFeatureSet = FeatureSet.OpenFile("Q:\cities.shp")
for each f as feature in fs.features
'here you would see all features
next
fs.DefQuery = "[Name] LIKE De%"
for each f as feature in fs.features
'now you have restricted the featureset to only those that meet the criteria
next
Dim lyr As MapPointLayer = New MapPointLayer(fs)
mapMain.MapFrame.DrawingLayers.Add(lyr) 'this would display only the features that meet the criteria since it is now essentially a new featureset
But the PolygonCategory approach works for me although the .Symbology.GetCategories.First.FilterExpression does not.~AGP