Stack
A Stack or Stacking is a mechanical term. It refers to multiple instances of the same or similar effects combining together. If two effects can Stack, you gain the benefits of both. Whereas, if something does not Stack, you gain the benefits of only one effect, not both. The [Active Player] chooses which of these effects apply.
All effects Stack unless they share the same name or the effect specifies otherwise.
Note: An Item's name is the name of its effect.
Example 1: Orik Blastshield the Dwarf gains 2 Armour due to their racial Ability Stoneskin. Orik also has a set of Medium Combat Armour Equipped that gains them an additional 4 Armour. Since both effects have different names they can Stack, giving Orik a total of 6 Armour.
Example 2: Orik has recently spent some [Experience] and gained the [Sturdy] Ability from the [Meat] [Mastery]. The [Sturdy] Ability gives Orik -5 to the Total of Injury [Tests] they generate. Since Orik is a Dwarf they also gain the [Sturdy] Ability from their racial Ability Stoneskin. Given [Sturdy] has the same name as [Sturdy] they do not Stack.
Example 3: Orik has been Attacked. The attacker has rolled a Total of 25 against Orik's 20 Defence making the Attack Successful. Orik then uses a Quick Action to perform the Dodge Action. This gives them +3 Defence for a total of 23 against the pending Attack. 23 Defence would not be enough to make the Attack Unsuccessful. Orik then uses their last Quick Action Point to perform the Dodge Action again. Since Dodge has the same name as Dodge they do not Stack and Orik gains no additional benefit. Orik's Defence remains at a total of 23, leaving the greater Attack Total of 25 to be Successful.
Wuntch's Whatever
While it is intended that most of the math in Meat Space and by extension, stacking, is done additively, there's some edge cases like mixing bartering and the Signature Range Ability where percentages make it a bit funky. You may need to have a discussion to decide what makes the most sense at the time. I'd recommend taking cases like this step by step to make life easier, just apply the signature range as per the rules, and then apply anything else one at a time. So long as everyone is happy with the outcome, whatever you decide to do will be chill.