It's been a while since I have used it, I just about quit art for over a year and only in the last month or so started working my way back into.
From what I remember you choose which products you want your work on(pillow cases, rugs, mugs etc etc) and these have a base price plus your 10% which is how much a customer pays for it. You can go above the 10% say 25% so if a print base price was £10 they would add your 10% on and charge £11 for it when a customer purchased it and you would get the £1. Obviously this isn't much but it you have art which isn't a commission piece it could make you some money without much additional work instead of it sitting in a folder. Once you have made the adjustments and put it in your store it can stay there indefinitely and potentially be making money without doing anything else with it(no postage, no dealing with customers, no extra time drawing to certain specific requests, with it being print to order no additional expenses on art supplies) it is a good way to be creative and express your own art without the extra pressures of doing art for a specific customer and still potentially make some money from it.
(I hope I have explained this ok, I was just about to go to bed when I saw your reply and question so I'm not on best form answering your question right now
).