Existing Wording
Logical ids are case sensitive. Ids are always opaque, and external systems need not and should not attempt to determine their internal structure. An id SHALL always be represented in the same way in resource references and URLs. Ids can be up to 36 characters long, and contain any combination of upper and lowercase ASCII letters, numerals, "" and ".".

Proposed Wording
Logical ids are case sensitive. Ids are always opaque, and external systems need not and should not attempt to determine their internal structure. An id SHALL always be represented in the same way in resource references and URLs. Ids can contain any combination of upper and lowercase ASCII letters, numerals, "" and ".".

Comments
A. Conflicts with the length definition of 64 in datatypes.
B. 36 characters is insufficient for a fully qualified ID: There are currently 3 resources whose names are 24 characters. If I use http://fhir.mayo.edu/rest/Person/1000003915 is 43 characters . Note: the number 1000003915 is NOT a made up number - it is the current MAX(NUM) of our person table. Likewise for http://fhir/mayo.edu/rest/Encounter/107193947 which is 44 characters

Grahame's Comments
need more clarification about which part is limited to 64 characters (tail, not the whole)

Disposition Comment
Should say 64