First of all, those of you who know where these pictures have been taken,
please keep quiet!
Assume a building has three elevators. They can be intelligently designed to service the various floors of the building in this fashion:
This one stops at even numbered floors only.

This one only serves odd numbered floors. So far so good.

The third, obviously, serves All floors
only!! Heh heh, we could teach some English to those Firangs :-)