Q: Is a stopper knot tied at the end of a rope to prevent the end from unraveling? ¶
A: Yes.
Q: Is a stopper knot tied outside another knot and around the standing part it can also be called a backup knot? ¶
A: Yes.
Q: Is a stopper knot tied at the end of a rope to prevent the end from slipping through another knot? ¶
A: Yes, or passing back through a hole, block, or belay/rappel device.