An IEntityEquipmentSlot is a slot in a player's inventory. That could for example be the mainHand or one of the armor slots.

It might be required for you to import the package if you encounter any issues, so better be safe than sorry and add the import.
import crafttweaker.entity.IEntityEquipmentSlot;

Getter	Type
Getter index	Type int
Getter slotIndex	Type int
Getter name	Type string

You can check if two IEntityEquipmentSlot objects are equal:
Returns a bool.

ZenScript
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slotOne == slotTwo;

The EntityEquipmentSlot Interface has 6 static Methods, each returning the given IEntityEquipmentSlot object.

ZenScript
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crafttweaker.entity.IEntityEquipmentSlot.mainHand();
crafttweaker.entity.IEntityEquipmentSlot.offhand();
crafttweaker.entity.IEntityEquipmentSlot.feet();
crafttweaker.entity.IEntityEquipmentSlot.legs();
crafttweaker.entity.IEntityEquipmentSlot.chest();
crafttweaker.entity.IEntityEquipmentSlot.head();