In SEARCH mode PSS calculates errors on source flux and position. These errors can be calculated at up to 3 confidence levels for both quantities. The fitting software used by PSS (written by Dick Willingale at Leicester University) supplies both lower and upper bounds on the source parameters. By default PSS returns the average of the two numbers for a given error -- this is the `symmetric' error mode. By specifying the ASYMMETRIC logical keyword true, PSS will output both lower and upper bounds to the output file.
Expert mode also allows the number and level of confidence levels to be specified using the FERL and PERL parameters.