This manual describes the C3D file as being composed of a number of 512-byte blocks of information.  Various data sections within the C3D file are aligned on multiples of 512 bytes and pointers to sections within the C3D file structure are generally stored as block counts.  The choice of a 512-byte block size for the low-level structure of the C3D file is a historical artifact due to the use of FORTRAN in the original PDP-11 programming environment.

The term record is used to describe individual units of information such as parameters and data samples that are stored within various sections in the C3D file.  Individual sections and records within the C3D file may cross 512-byte block boundaries.