Battle tier
The tier bracket a battle is fought at, which is what decides who a vehicle meets rather than its own tier alone.
Also known as MM spread, tier spread, matchmaking tier.
Every vehicle has a range of battle tiers it can be pulled into, normally its own tier and the two above. A Tier VIII therefore sees battles at VIII, IX and X, and the position it holds in the list changes what its job is.
Some vehicles have a deliberately narrower spread. Most premium vehicles and a few researchable ones see only one tier above their own, which is why they are described as having preferential matchmaking.
Bottom tier is not a penalty box. A vehicle two tiers down cannot trade shots with the top of the list, so its value moves to spotting, flanking and finishing, which is exactly the shift the matchmaker's template forces on two thirds of every team.