Gaye,
I'm sure that they will answer you. If you sell a photo by copyright, you no longer own the photo and should be paid the big bucks. Like in the mega K range. One time use is for a specific job/publication. I'm butting in here because there are other terms to know. Most times I will do an informal contract by email but have gone the more formal in print route too. I always specify non-exclusive use in the wording so that there is no mistake that the client agency understands that the same photo may show up elsewhere. I have photos that I have sold many times. Exclusive Use means I still own the photo but I can't resell use. Cuts the price between non-exclusive use and copyright sales. No reputable agency or client has ever griped about non-exclusive use since they favor the lower cost.
A gated community agency called me two weeks ago asking if I had a specific photo for them to use on their web site. I do and I told the lady that I had the shot because an ad agency in North Carolina that represented this same caller (new person) purchased the use of the photo a couple of years ago. The lady thought that she had seen the photo in something but couldn't find a file copy of it. That told me something important and I told her it was because the ad agency held the photo as it was a one time use that was paid for. As you can guess, the lady caller does not know much about photo use and sales so I'm grateful to the ad agency for their honesty. The lady ended up buying 8 photos non-exclusive for their web site. I, on my own, added in the email contract I wrote, no time limit on use. Someday they will want to update the photos anyway.
I see by email notification that someone has probably answered your question while I'm typing but I'll let this fly anyway.
Guess I better modify one thing after reading Gordon'd note. I suppose that Exclusive Use and Copyright Sale could be the same thing. Either way, that would be nice to see those big bucks!!