Threat is an aggregate statistic that gives a general summary of a Hero's other main stats. Team Threat is a similar number, but aggregates the threat of every hero on a current team. Roster Threat (also known as Total Team Threat or TTV) adds up the threat values of all unlocked heroes in your Roster, which is displayed in Leagues and often used as a metric for a player's progression in the game.
It seems to take into account a Hero's Health, Defense, Blocking, Critical Attack Resistance, Stun Resistance, Attack, Critical Attack Chance, Critical Attack Damage, Fast Attack Chance, Lethal Attack Chance, Armor Piercing Chance, passive and level of special attacks. It may go up or down if you switch between calculated and uncalculated stats (such as when rerolling a talent and switching from attack to DOT resist or hero XP), or if you switch between stats that are weighed differently.
Team threat is not a simple addition of the threat level of every individual member on your team. There are two different equations depending on the number of cards in the team.
For a 2 card team, team threat is calculated as 1.25 x higher card threat + 0.75 x lower card threat - 1.
For a 3 card team, team threat is calculated as 1.5 x highest card threat + 0.85 x middle card threat + 0.65 x lowest card threat - 1.
Roster Threat, Roster TTV, or Roster Total Threat Value, is the sum of all your characters' threat values. This is displayed in leagues, the Roster page, as well as the "Player Account" interface.
Individual Hero Threat
Threat for each individual hero is calculated on the basis of a trinomial with three simple groupings:
- Health * hero multiplier * (defense component + blocking component + resistance component) + Damage * hero multiplier * (crit attack chance component + fast attack chance component + lethal attack chance component + armor pierce chance component) + Attack * hero multiplier * (passives component + special attack component) + Attack * hero multiplier * artifact level (if artifact has been equipped in the artifact slot)
The hero multiplier is a factor applied to each part of the equation which varies from one hero to another. The multiplier has a base value of 1 for some of the older heroes and is higher for some of the recently added heroes. All legendary heroes have a hero multiplier of 1.2.
Each component has a specific weightage and increasing one component will result in a greater threat gain compared to another. Depending on how the hero is to be built as, the corresponding factors are to be given priority. If he/she is meant to be a tank, adding more health, defense and resistance would be suitable. For a damage dealing hero, adding more attack along with critical attack chance / critical attack damage / fast attack chance / lethal attackchance / armor pierce chance would make more sense.
A stats and threat calculator sheet can be found here
As both threat and team threat are aggregate numbers, they are statistics that should be taken with a grain of salt when assessing your own team, as you could have several choice stats (as from high level talents) on your character that you value (like DoT resist on a "tank") that do not figure into the calculation. You can also weigh your strongest heroes in most situations more heavily than the game does, as your damage output is somewhat a function of how active your most offensively powerful character is in a given match.
In several locations like Campaign mode, however, where you cannot view the stats of individual heroes on the opponent's team (as you can in arena), the opponent's team threat level may be the only way you can assess the opposing team's strength. You can, however, discount the opponent's team threat to some degree when it includes multiple weak opponents like the underlings in campaign mode, as this means the opponent's average true threat is lower.
If you purposefully want to decrease or "mask" threat level for whatever reason (easier opponents in multiplayer, perhaps), you can leave abilities that you don't assign value to with fewer (if any) upgrades.
Operations require a minimum threat value for a hero, from 1 to 4000. This is generally only relevant to very early game, as many gold heroes are unlocked at 3000 or more threat, even at level 1 and without gears, talents, or ability upgrades.