Resume IDs are used when you want to re-upload the same item to space but don't want to duplicate multiples of the same item in your inventory or in the shop.
You can find the items' ID in your Curator Items panel.
Once you have this number written down, Open Unity.
When Unity is open, find the item you took the ID from, for example, mine was Long Shorts.
In the Inspector window, you will see all of the information you have completed already for the item.
You will need to scroll in the Inspector window until you see this.
Enter the ID you just noted down, and put it in the Resume ID box. Once this done, click on 'Apply Resume ID'.
Scroll down to Automatic Submission, and this will upload the same item with the same ID!