How does Sherpa handle Level=3 PHA spectra downloaded from the Chandra Source Catalog?

Each Chandra Level=3 PHA file (pha3.fits) available in the Chandra Source Catalog contains both a source and background spectrum in separate FITS HDUs (CXC Data Model blocks). When a pha3.fits file is loaded into Sherpa with load_data or load_pha, the background spectrum is automatically recognized and read in as well, with the same filename as the source spectrum. The resulting source and background Sherpa data sets may be handled in the usual way in Sherpa.

sherpa> load_pha('acisf06402_000N022_r1269_pha3.fits.gz')
read ARF file acisf06402_000N021_r1269_arf3.fits
read RMF file acisf06402_000N021_r1269_rmf3.fits
read background file acisf06402_000N022_r1269_pha3.fits

sherpa> show_all()

Data Set: 1
Filter: 0.0110-14.9431 Energy (keV)
Bkg Scale: 0.0463094
Noticed Channels: 1-1024
name           = acisf06402_000N022_r1269_pha3.fits.gz
channel        = Float64[1024]
counts         = Float64[1024]
staterror      = None
syserror       = None
bin_lo         = None
bin_hi         = None
grouping       = None
quality        = None
exposure       = 87299.461130877
backscal       = 1.249834895134e-06
areascal       = 1.0
grouped        = False
subtracted     = False
units          = energy
rate           = True
plot_fac       = 0
response_ids   = [1]
background_ids = [1]

RMF Data Set: 1:1
name     = acisf06402_000N021_r1269_rmf3.fits
detchans = 1024
energ_lo = Float64[900]
energ_hi = Float64[900]
n_grp    = UInt64[900]
f_chan   = UInt32[1029]
n_chan   = UInt32[1029]
matrix   = Float64[401413]
offset   = 1
e_min    = Float64[1024]
e_max    = Float64[1024]
ethresh  = 1e-10

ARF Data Set: 1:1
name     = acisf06402_000N021_r1269_arf3.fits
energ_lo = Float64[900]
energ_hi = Float64[900]
specresp = Float64[900]
bin_lo   = None
bin_hi   = None
exposure = 87299.461130877
ethresh  = 1e-10

Background Data Set: 1:1
Filter: 0.0110-14.9431 Energy (keV)
Noticed Channels: 1-1024
name           = acisf06402_000N022_r1269_pha3.fits
channel        = Float64[1024]
counts         = Float64[1024]
staterror      = None
syserror       = None
bin_lo         = None
bin_hi         = None
grouping       = None
quality        = None
exposure       = 87299.461130877
backscal       = 2.6988796889782e-05
areascal       = 1.0
grouped        = False
subtracted     = False
units          = energy
rate           = True
plot_fac       = 0
response_ids   = [1]
background_ids = []

Background RMF Data Set: 1:1
name     = acisf06402_000N021_r1269_rmf3.fits
detchans = 1024
energ_lo = Float64[900]
energ_hi = Float64[900]
n_grp    = UInt64[900]
f_chan   = UInt32[1029]
n_chan   = UInt32[1029]
matrix   = Float64[401413]
offset   = 1
e_min    = Float64[1024]
e_max    = Float64[1024]
ethresh  = 1e-10

Background ARF Data Set: 1:1
name     = acisf06402_000N021_r1269_arf3.fits
energ_lo = Float64[900]
energ_hi = Float64[900]
specresp = Float64[900]
bin_lo   = None
bin_hi   = None
exposure = 87299.461130877
ethresh  = 1e-10