COPW will copy a wavelength calibration from one spectrum to another. This is done to copy the wavelength scale computed with the WSCALE command from the comparison spectrum to a program spectrum. The actual spectral data are not changed by this command, only the wavelength calibration parameters stored in the image header.
If a second source spectrum is given then the command assumes that you have two comparison spectra which bracket the program spectrum in time, and it will use the times of observations of the image headers of the three spectra to do a linear interpolation of the wavelength parameters of the comparisons to the time of the program spectrum. This procedure can sometimes provide an improved wavelength calibration by correcting for the effects of instrumental flexure.