Data Organization: II. Organizing Tracks by Block
Rather than being divided into sectors, the disk tracks may be divided into user-defined blocks.
When the data on a track is organized by block, this usually means that the amount of data transferred in a single I/O operation can vary depending on the needs of the software designer (not the hardware).
Blocks can normally be either fixed or variable in length, depending on the requirements of the file designer and the capabilities of the operating system.