What is the difference between a combat and a showdown, and can a showdown occur without a combat?

Ruling: A showdown occurs whenever a unit moves into a battlefield. A combat occurs when units controlled by opposing players are present at the same battlefield. A showdown can occur without a combat when moving to an empty battlefield.

Sequence:

• Move to empty battlefield = Showdown without combat
• Move to occupied battlefield = Showdown with combat

Nuances:

• A showdown is not derivative of combat; it is a part of combat when combat occurs, but can also exist independently (like how a wheel can be part of a car but also of a bike)