Currently, chip can be "hgu95av2", "hgu133a", "hgu133plus2", or "u133x3p". If no further arguments are specified, the scores for all probe sets on the chip are returned.
If any of probeset, eg, symbol, alias, or ensembl are specified, these are used to filter the resulting data frame in a logical OR sense.
Details about the jetset algorithm are available in the vignette.
References
Qiyuan Li, Nicolai J. Birkbak, Balazs Gyorffy, Zoltan Szallasi and Aron C. Eklund. (2011) Jetset: selecting the optimal microarray probe set to represent a gene. BMC Bioinformatics. 12:474.