MAS 5.0 is the name of the Affymetrix algorithm used for producing gene expression signal (see Affymetrix WebSite) for more details on the algorithm.
The algorithm consists of background correction, calculation of the probe summary and scaling (typically termed probeset-level normalization). The options available within GENE include:
Finally, you can select the name of the output file to write
expression results. This file is a tab-delimited output file with
columns labeled with CEL file names (without the .CEL extension) and
rows corresponding to probesets. The default is exprs-mas.txt
in the working directory.