The 'trace-width-threshold' is provided for cases when the S/N of the frame being examined to locate the orders is very low.
The threshold is the degree to which off-trace pixels must fall below the pixel at trace centre (in the spatial direction) in order to be considered as 'off-order'. I.e., the trace width is estimated by stepping pixel-by-pixel above and below the trace until the intensity falls below (1.0 - TUNE_TWTHR) * intensity-at-trace-centre.
The default value is 0.95 (95%).
Martin Clayton