Concerning ISBN numbers - what does a "block of 10" mean?

It costs over $300 for a block of 10 ISBN numbers. Does this mean I can purchase numbers for only ten books? If so, how can any publisher afford such a thing when they publish hundreds of books a year?|||The ISBN number only differs for each different edition of a book, not each different copy, so you could print 1000 copies of a new hardcover book with one of those ten ISBN numbers... if you wanted to print 500 paperback copies, you'd have to use a new ISBN number for those.