I believe this is editorial insofar as I believe the working group intended to give these statements normative strength, even though the appropriate keywords were not used.

4.3 SML Reference Schemes
from: It is not required that all elements in an SML model be reachable
to  : It is OPTIONAL     that all elements in an SML model be reachable

5.2.1.2 Schema Component Rules (for identity constraints)
from: The sml:selector XPath allows  smlfn:deref()
to  : The sml:selector XPath MAY use smlfn:deref()
...same for sml:field, immediately following

6.3.2 Schema Component Rules (for embedded rules) items 2 and 3
from: they are     automatically copied
to  : they MUST be automatically copied

from: in B cannot   be restricted to a local
to  : in B MUST NOT be restricted to a local