It is a certificate for EU citizens taking extended vacations & stating that you are a tourist , not resident , not fiscally resident & that there's no requirement for you to register as you are a resident of another EU state.
I mentioned motorhomes as that is the place where the question usually cropped up & where many can explain exactly what it is called & the easiest place to obtain it.
If you were doing any form of studying then that would be another way of bypassing the registration requirement as students/scholars on placements in other EU states have up to 2 year exemptions from any requirement to register & can bring all goods & chattels . ( including motor vehicles without any requirement to re-register them .)
Personally & knowing what I know, I wouldn't voluntarily register for anything if I was only staying for one year & not returning .
Once in the system any inadvertent omission of something could lead to them attempting to notify you but you've already gone back to the Uk; They'd then put it in the BoE , Which is the only legal requirement they actually have to do !
Remember , in Spain it is up to you to check regularly that you don't owe anyone/anything/have forgotten to pay/ forgotten to tax/itv the car , etc; etc, There is no requirement , other than them placing it in the BoE , to notify you.
If this happened & once the BoE has been has been placed they can then apply for a European Court Order for the debt, etc; Once granted this gives them the power to seize/block/sell all assets that you own in your Resident country ,the UK, & subject to applying to the UK for a court order ,which unfortunately will be automatically granted by virtue of the Spanish EC order.
Better to be fined for non registering in my book.
or a bit of reading, asking questions and you can do it yourself.......... 
