S rate of Gibraltar stamp


Dear my friends, now I need your help. G on a Gibraltar stamp means the stamp can be attached on an envelope or postcard from Gibraltar to Gibraltar and E means from Gibraltar to Europe. But I browsed internet again and agian, no website tells me how to use S stamp of Gibraltar?
Thank you.
Robert Lee
Beijing, China

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Spain?

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Surface ?

They do seem to have a separate Spain category/rate so that is probably it…