Existing Wording: Profiles cannot break the rules established in the base specification (e.g. if the element cardinality is 1..1 in the base specification, a profile cannot say it is 0..1, or 1..*)

Comment:

What is and what is not allowed for cardinality overrides in derived profiles is not sufficiently defined in the specification.

"Limitations of Use" section in profiling page only gives example of 1..1 => 0..1 or 1..* not allowed, but does not give other restrictions; e.g. 0..1 => 0..*, etc. A number of FHIR profiles were created that break the rules and this will continue without a clear clarification of the rules and how they should be interpreted.

The table listed in FHIR blog, however, gives a table with rows showing cardinality for all possible values in the domain resource element against each column being cardinality specified in a derived profile. This table makes it explicit what is and what is not allowed given the base specification.

Source: http://fhirblog.com/2014/03/26/fhir-profiles-an-overview/

FHIR spec should include the table as described in the referenced blog (or equivalent) to give a more complete description of what is and what is not allowed.