Objective-C use property and it's storage location. Difference?

Question

What is the difference between property name and property storage location? I must use only property name or mm what is the point to use first or the second? Here is my example:

Answer 1

self.carSpeed = speed; 

This uses setter method

Whereas,

_carSpeed = speed;

uses directly the ivar or property through the alias created.

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Using . notation gives you the facility to access them from outside the class and setters and getters are in public zone.

Whereas, _carSpeed makes it local to the class. And more it is a old convention.

Synthesized property and variable with underscore prefix: what does this mean?