Wavelengths are stored in the ctype field and determine the colour of the elements (e.g. PHOT, FILT, BRAY, BIZR, BIZS and BIZG.) They can be used as storage by setting the wavelengths to a certain amount and read that info by filtering out some wavelengths and letting others through. https://powdertoy.co.uk/Browse/View.html?ID=2332971 shows that use.