The backscale value records the ratio of the data extraction region area to the total detector area for a PHA spectrum file. If the background scaling is independent of channel, it is written to the BACKSCAL header keyword; otherwise, it is recorded in a BACKSCAL column.
In this example, the extraction region is 6283.19 square pixels. Dividing by the total ACIS detector area (8192x8192), yields the backscale value:
6283.19/(8192*8192) = 9.36267e-05
which is the value recorded in the file header:
unix% dmkeypar simple.pi BACKSCAL echo+ 9.3626757073098e-05