The tubes are referred to as the tires to be assembled.
Thus 4.00-6 tube is designed to be mounted on a 4.00-6 tire. Some cameras can be mounted in various sizes of tires, which is indicated in the product specifications.
The nomenclature is interpreted as follows: The first digit indicates the height of the camera from the rim to the deck in inches, and the second digit indicates the rim diameter in inches.
So 4.00-6 tube is 4 inches tall and is designed for 6 inches rims.