What do the different emergency vehicle sirens mean?

a motor machine used for transportation.

all traffic, in either direction, must slow down, pull to the right of the road, and stop until the emergency vehicle passes

It means "not the same" Example: Humans and Dogs are different species

Only when instructed by a senior ranking officer (be it fire/ambulance or police)

to be alone mean to have some thing miss inside even if you have many people around you