The background fits files are saved as defined by the OGIP Calibration Memo CAL/GEN/92-007.

It contains 2 columns called: CHANNEL and COUNTS. The channel to energy conversion is contained in the response matrix file, rmf.fits. Following HEFT convention though it has been set to 15 channels per kev. 

The background files include the diffuse and internal background, and there is a set for the 0 deg and 27 deg orbit. Additionally there are files with different exposure times, 1e4 sec, 1e5 sec and 1e6 sec. 

The background scaling is set in the header keyword, 'backscale' and it is in arcmin^2/cm^2. When loaded into XSPEC, xspec will divide the background counts with the 'backscale', and thus since the raw counts are in counts/cm^2 the output spectrum will be counts/arcmin^2.

If a different scaling is required, the 'backscale' keyword in the header can be changed manually.

The .dat files are the text file versions and is the combined background of diffuse and internal. Read the 'COMMENT' part of the corresponding fits file header for more info on what is included.