In the latest version of the Nostr Protocol, there have been some changes made to the way labels are used in the protocol. The new `l` tag denotes a label and the new `L` tag denotes a label namespace. A label must include a mark matching an `L` tag. The `L` tag refers to a tag type within nostr or a nomenclature external to nostr defined either formally or by convention. Any string can be a namespace but publishers should ensure they are unambiguous by using a well-defined namespace such as an ISO standard or reverse domain name notation. This is a way of attaching standard nostr tags to events, pubkeys, relays, urls etc.

Link: https://github.com/nostr-protocol/nips/blob/master/32.md