Flag values are generated by SExtractor during the generation of 
photometry data. The flag value is created by treating each separate flag condition 
as a bit flag, with the overall value determining which flags are enabled or disabled. 
The bits and conditions are organized in order of increasing concern, starting at bit 0.

The following table shows the bit number and value that a single condition contributes
to the extractor flag value. Note that this means more than one condition can be reported
by the flag value.

Bit | Value | Meaning
0   |   1   | Aperture photometry is likely to be biased by neighboring sources or by more than 10% of bad pixels in any aperture
1   |   2   | The object has been deblended
2   |   4   | At least one object pixel is saturated
3   |   8   | The isophotal footprint of the detected object is truncated (too close to an image boundary)
4   |  16   | At least one photometric aperture is incomplete or corrupted (hitting buffer or memory limits)
5   |  32   | The isophotal footprint is incomplete or corrupted (hitting buffer or memory limits)
6   |  64   | A memory overflow occurred during deblending
7   | 128   | A memory overflow occurred during extraction


For example, a value of 1 indicates that the condition associated with bit 0 has been flagged.
Another example: a value of 3 indicates that the conditions associated with bits 0 and 1 have been flagged.
As a final example, a value of 70 indicates that the conditions associated with bits 1, 2, and 6
have been flagged. 

The above is based on original documentation in 
E. Bertin and S. Arnouts. SExtractor: Software for source extraction.
A&AS, 117:393–404, June 1996. doi:10.1051/aas:1996164