Unique ID
Unique IDs, or item IDs, identify an item with a project key, set key and a set counter. Unique IDs remain the same even if item name or content is changed. They can't be edited or duplicated.
Project key
A project key identifies which project an item belongs to. Project keys must be unique to each project, and must be 1–16 characters long, consisting of letters, numbers, or underscores. A project or organization administrator can change the project key, but that doesn't reset the numbering of items.
Set key
A set key identifies item sets for each project. You can use the same set key across sets or projects. Project or organization administrators can change a set key to better meet your organization's needs.
Set counter
The set counter is a number automatically added to the item. Unlike the Global ID counter, you can reset the set counter when you change the set key.
Note
It's possible to use the set key across multiple sets of the same item type. However, keep in mind that the set counter is shared across all items using the same set key.