Good morning. The selection mask triples allow you to specify what types and subtypes of objects can be selected.
In the Solution Center, search for:
nxopen sample selectionmanager
Then when the initial list of hits is returned, on the left side of the page, under Document Type, click on "nx_api". This will show you 20 small examples that use the SelectionManager.
"Types" and "subtypes" help to categorize NX objects. Every NX object will belong to a certain type and many categories will be further divided into subtypes. Using one or more mask triples will allow you to filter the user selection for the type(s) of objects you are interested in. For example: does your journal only work on sheet bodies? You can set the mask triples so that the user can only select sheet bodies (not solid bodies, curves, datums, etc).
In addition to the GTAC examples, you might also look at:
The article was written with NX 7.5 in mind; if you use NX 8 or newer, use the function .SelectTaggedObject or .SelectTaggedObjects instead of the deprecated .SelectObject or .SelectObjects.