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UMI’s stacking system is sophisticated, allowing different items to stack differently based on their properties and use case.

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Stacking Policies

1. CommodityDecayAware Use Case: Food, perishables, items that decay over time How It Works:
  • Groups items into “freshness buckets” based on durability/age
  • Bucket width controls granularity
Example (20% buckets):
  • Bucket 1: 100% - 81% fresh → Stack together
  • Bucket 2: 80% - 61% fresh → Stack together
  • Bucket 3: 60% - 41% fresh → Stack together
  • Bucket 4: 40% - 21% fresh → Stack together
  • Bucket 5: 20% - 0% fresh → Stack together
Configuration: 
Stacking Policy: CommodityDecayAware
Bucket Width Percentage: 20
Max Stack Size: 50
Result: You can have multiple stacks of the same food at different freshness levels. 2. GearDurabilityBucket Use Case: Equipment, tools, weapons with durability How It Works:
  • Similar to CommodityDecayAware but for gear
  • Groups by durability percentage
Example (25% buckets):
  • 100% - 76%: Pristine gear stacks together
  • 75% - 51%: Good condition stacks together
  • 50% - 26%: Worn gear stacks together
  • 25% - 1%: Damaged gear stacks together
Configuration: 
Stacking Policy: GearDurabilityBucket
Bucket Width Percentage: 25
Max Stack Size: 10
Result: Fresh swords stack separately from damaged swords. 3. ProvenanceMatters Use Case: Crafted items where the crafter matters How It Works:
  • Includes crafter name in stack key
  • Items from different crafters don’t stack
Example:
  • “Iron Sword crafted by Thorin” (5 items)
  • “Iron Sword crafted by Elena” (3 items)
  • These occupy separate stacks even though they’re the same item
Configuration: 
Stacking Policy: ProvenanceMatters
Max Stack Size: 20
Result: Player-crafted items maintain their creator’s identity. 4. FullInstance Use Case: Unique items, quest items, items with unique properties How It Works:
  • Each item is completely unique
  • Never stacks with anything
Example:
  • Legendary weapons
  • Quest items
  • Items with random enchantments
Configuration: 
Stacking Policy: FullInstance
Max Stack Size: 1
Result: Each item occupies its own inventory slot.

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Stack Keys

Stack keys determine if two items can merge. Items with identical stack keys will stack together (up to MaxStackSize). Stack Key Composition:
  • Item ID
  • Item Data reference
  • Rarity (if bIncludeRarityInStackKey is true)
  • Policy-specific data:
    • CommodityDecayAware: Freshness bucket
    • GearDurabilityBucket: Durability bucket
    • ProvenanceMatters: Crafter name
    • FullInstance: Unique instance ID

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Choosing the Right Policy

Item TypeRecommended PolicyWhy
FoodCommodityDecayAwareTrack freshness
PotionsCommodityDecayAwareSimple commodities
Ore/ResourcesProvenanceMatters or CommodityStack easily
WeaponsGearDurabilityBucketSeparate worn from new
ArmorGearDurabilityBucketTrack condition
Unique GearFullInstanceNever stack
Quest ItemsFullInstanceOne-of-a-kind
AmmoCommodityDecayAwareSimple stacking

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Rarity and Stacking

By default, items of different rarities will stack together unless you enable bIncludeRarityInStackKey. Example: 
bIncludeRarityInStackKey: false
→ Common Iron Sword + Rare Iron Sword = Stack together

bIncludeRarityInStackKey: true
→ Common Iron Sword and Rare Iron Sword = Separate stacks
When to enable:
  • When rarity significantly changes the item
  • When you want players to easily identify item quality
  • For loot-heavy games with many rarity tiers
When to disable:
  • When rarity is cosmetic only
  • When you want items to stack regardless of quality
  • For simple games with limited rarity impact