The SRTM X-SAR Error Mosaic is based on the height error map (HEM, see SRTM PDF http://www.dlr.de/eoc/Portaldata/60/Resources/dokumente/7_sat_miss/SRTM-...) and provides a local measure of the achieved accuracy. It is statistically determined from a neighborhood of image cells mainly considering the phase and baseline stability. Thus it describes the precision relative to the surrounding. The determination of the absolute accuracy requires the consideration of reference measures.
Horizontal accuracy (absolute): ±20m 90% circular error
Horizontal accuracy (relative): ±15m 90% circular error
Vertical accuracy (absolute): ±16m 90% linear error
Vertical accuracy (relative): ±6m 90% linear error