I don't recall there being any such thing as a LOWER airway. The prefix L would normally indicate a Low or Medium Frequency airway. Prefix U is normally associated with that portion of the airway which is RNAV. There are others too: G is for Flight Information Service only, DOM for domestic traffic, D for direst, V for victor or VHF, and then NAT, OTR, R and so on. The names can also have meaningful suffixes. Check out your Jeppesen Introduction section. This stuff used to be in the pages numbered fifty-something.