Supposing you have a table Vehicle, you might need tables Truck and Motorcycle to hold information specific to only some of the rows in Vehicle.
You're describing here a design rule able to emulate an heritage, which is mostly a matter of methodology/dba experience.
With a (real) OODBMS, this kind of
trickery relational modelisation is not required.
My two favorites:
If the only tool you have is a hammer, you will see every problem as a nail. --Abraham Maslow
Bien faire, et le faire savoir...