These are simply words with headers local to the file they've been defined in.
As such, their scope is limited to that file.
They serve mostly the purpose of not cluttering the dictionary with single purpose words, using up precious word naming potential: All headers marked as transient will be discarded, only the "interface" type words remain.
This feature is of course nestable across nested files loading: each set of transient words is local to the file they've been defined in.

Example of using transient