[Matt Goelz]

sorry i didnt notice this was already responded to, but this explains the mechanum and holonomic drive situation in a little more depth if needed.

Its both actually. This understanding comes from any holonomic drive that uses omni wheels. Each wheel has 2 directions it wants to go. The way the system goes one direction specifically is by each direction canceling out with an opposite direction from the adjacent wheel except the direction it will travel. You can only achieve this type of drive that uses this implementation with wheels that slide easily or omni wheels.

A mechanum or a holonomic drive where the wheels are angled in the four corners in a diamond pattern for example:
the front right wheel wants to go forward and right
the front left wants to go forward and left
the back right wheel wants to go forward and right
the back left wheel wants to go forward and left

This results in the system moving forward because the lefts and rights canceled out. To be able to cancel out directions like this, you have to use a very low friction wheel or omni wheel. In theory, if you have wheels that slide very easily, they will slip on the floor and move the robot. I have never actually seen this done, as all the holonomic drives I've seen use omni wheels.