Some specific properties of some URI schemes
scheme class of that identified Notes, supported protocol Authority/disambiguation
http document Get and put. Can take #fragid DNS
service End point, send or send/receive - does not take #fragid
ftp file get and put

(Incomplete architecturally: file format is guesswork)

mailto: email address ("mailbox") End point, send. (Also occurs in many other methods in eg IMAP) DNS
uuid: anything unique, not reused MAC address (IEEE etc)
mid: email message @@ Can use for anything?

@@ Identify body document separately?

cid: attachment @@ anything? @@ body? see mid: DNS
md5: file of octets No protocol. Directly verifyable. None
tel: telephone number for speech Human conversation ITU E164 and national regulators
fax: telephone number for fax End point, send to only. ITU E164 etc
urn: anything. Various, subscheme-dependent. No general protocol support. IANA, then subscheme-dependent

The table demonstrates the diversity of properties of URI schemes, indicating both that the URI system itself was deigned to be very general and allow the craetion of new schemes, but also that a wide range of schemes do exist and should be used rather than re-invented.

@@ represent TAG-relvant questions.