Attachment slots are red-marked points located on platforms, vehicles, the player's backpack, and some modules, that most items can be attached to. They are used to store items, power Modules, or secure items for transport, among other uses.
- Small, or Tier 1: Small slots are single slots, that can only hold one Small item. Examples of items that fit in these slots are Resources and Augments.
- Medium, or Tier 2: Medium slots are made of two Small slots connected together. These slots hold Medium items, but may also hold two single Tier 1 objects. Examples of items that fit in these slots are RTG, Medium Printer, or a Medium Storage Silo.
- Large, or Tier 3: Large slots are made up of two Medium slots connected together. These slots hold Large items. Examples of items that fit in these slots are Smelting Furnace, Large Shredder, or Large Resource Canister.
- Extra Large, or Tier 4: Extra Large slots are made up of two Large slots connected together. These slots hold Extra Large items. Examples of items that fit in these slots are Extra Large Shredder, Extra Large Storage, or an Auto Extractor being transported.
- Auxiliary Slots: Auxiliary, or Aux slots, are special slots that are only found on the Backpack. These are the two top slots that are used to hold certain widgets that the player can activate with their context keys (PC: C or V, XBOne: or, PS4: or ), such as Extenders, a Portable Oxygenator or a Worklight.
- Reaction Slots: These slots appear on a few modules and items, such as the Medium Sensor Arch, Large Sensor Hoop A, Large Sensor Hoop B, Large Sensor Ring and XL Sensor Arch. They act differently than normal Small slots. When a player or certain items pass through the rings or under the arch, it will trigger the slot to activate whichever item is on the slot, such as Fireworks or a Small Camera.
- Input Slots: Printers, generators, and the various crafting modules have input slots which only accept a given set of resources, which may depend on a selected recipe.
- Output Slots: Some of the crafting modules also have a dedicated output slot where the module's resource output is stored unless there is an empty, regular slot available on the connected platform or vehicle. The player may take the resource item out of the slot, but cannot put another one in its place.
Attachment slots have a few uses in the game. Their main use is for providing power to connected modules, and for helping to expand the player's storage capabilities. If a player wishes to store items without risk of being lost, then holding them in slots will keep the items in place. Slots are also used on modules to craft items or other resources.
Other uses include using reaction slots to play games with friends, or to have races using the reaction slots on Large Sensor Hoop A. Slots on the Terrain Tool can modify how the player interacts with terrain through the use of Augments.