The bcf directory contains "Basic Calibration Files" and the cpf directory contains "Calibration Product Files"

Basic Calibration Files contain basic calibration information such as 
	the effective area, energy resolution, point-spread function 
	of the instrument. In many ways these datasets can be thought of 
	as the 'atomic' units of calibration.

Calibration Product Files contain 'higher' level calibration products
	(which can usually be generated using BCF datasets) suitable 
	for specific data analysis tasks. Examples include exposure maps,
	response matrices etc.