Hi Bob,
The intent of the UHC-E filter is to improve viewing of certain deep sky objects (eg. nebulae and supernova remnants), especially from light polluted suburban skies. The important wavelengths for these targets tend to be the Hydrogen Alpha, Hydrogen Beta, and Oxygen-III spectral lines.
By filtering out wavelengths from common light pollution sources (eg. sodium vapour and mercury vapour street lamps), the background sky is appears darker which which improves contrast for viewing details of the nebula or supernova remnant.
Attached is a link to info page at Astronomic:
http://www.astronomik.com/english/eng_uhc-e.html
Cheers,
Jeff