Like many FPS games, Overwatch uses its own native sensitivity settings along with a users mouse DPI to allow a character to produce movement.

Players may already have an optimal distance in which they move the mouse to complete one 360 degree character rotation derived from a different game. Therefore, in order to keep things consistent, they can use that distance calulation along with their mouse DPI to achieve a comparable in-game sensitivity for Overwatch.

Your mouse DPI can be found in whichever software configured your mouse. The sensitivity equalizer is used for converting a cm/360 result from a different game into a native Overwatch sensitivity.

It is expected that you have this information, whether it be from logging into whatever game you're playing and physically measuring the distance it takes to move your character, or using another conversion system like shown on this application.