PM,
They are source indicators, on our aircraft the selection of GPS/VLoc is made through the GNS430.
Press the CDI Button to select GPS or VLOC.
The Nav/VLoc selection is made through the frequency used in the Nav receiver.
(We have VLOC,NAV & GPS indicators on ours).
TLAR
NTB
PS I agree, not a great manual the clues are implied