Information graciously supplied through URW, via Vincent Ertner: Each URW font has an assigned font number. The last character describes the size or purpose of the font, as the design of the font is optimized accordingly:
X000000EX000000TX000000DX000000PX000000MX000000LX000000IThe urw.map file has abbreviations for URW fonts.
uagd8a URWGothicL-Demi a010015l
uagdo8a URWGothicL-DemiObli a010035l
uagk8a URWGothicL-Book a010013l
uagko8a URWGothicL-BookObli a010033l
ubkd8a URWBookmanL-DemiBold b018015l
ubkdi8a URWBookmanL-DemiBoldItal b018035l
ubkl8a URWBookmanL-Ligh b018012l
ubkli8a URWBookmanL-LighItal b018032l
ucrb8a NimbusMonL-Bold n022004l
ucrbo8a NimbusMonL-BoldObli n022024l
ucrr8a NimbusMonL-Regu n022003l
ucrro8a NimbusMonL-ReguObli n022023l
uhvb8a NimbusSanL-Bold n019004l
uhvb8ac NimbusSanL-BoldCond n019044l
uhvbo8a NimbusSanL-BoldItal n019024l
uhvbo8ac NimbusSanL-BoldCondItal n019064l
uhvr8a NimbusSanL-Regu n019003l
uhvr8ac NimbusSanL-ReguCond n019043l
uhvro8a NimbusSanL-ReguItal n019023l
uhvro8ac NimbusSanL-ReguCondItal n019063l
uncb8a CenturySchL-Bold c059016l
uncbi8a CenturySchL-BoldItal c059036l
uncr8a CenturySchL-Roma c059013l
uncri8a CenturySchL-Ital c059033l
uplb8a URWPalladioL-Bold p052004l
uplbi8a URWPalladioL-BoldItal p052024l
uplr8a URWPalladioL-Roma p052003l
uplri8a URWPalladioL-Ital p052023l
usyr StandardSymL s050000l
utmb8a NimbusRomNo9L-Medi n021004l
utmbi8a NimbusRomNo9L-MediItal n021024l
utmr8a NimbusRomNo9L-Regu n021003l
utmri8a NimbusRomNo9L-ReguItal n021023l
uzcmi8a URWChanceryL-MediItal z003034l
uzdr Dingbats d050000l