Difference between .m4a and .m4b files

Am trying to load audiobooks into my new iPod Classic. Have checked off Media Kind: Audiobook, but the file is still listed as a .m4a file. Does it matter? What is the difference? Each audiobook may have upwards of 100 "tracks"....is it worth my time to manually change each one to a .m4b in Windows Explorer under the path: iTunes\itunes music\(author)\(title of book)\

Any suggestions for a cleaner way to import audiobooks for an iPod classic would be appreciated too.