This involves sml:target* and SML identity constraints.

There are 3 kinds of <xs:element> elements in schema documents:
1 For global element declaration
2 For local element declaration
3 For reference to global element declaration

On which of the above cases are sml:target* and sml:key/keyref/unique allowed to be specified? And to which schema component(s) (element declaration or particle) are these constraints attached to?

If these constraints are attached to element declarations, then they should not be allowed on case #3; if they are attached to particles, then they should not be allowed on case #1. So if we want to allow them being specified in all 3 cases, then the constraints need to be attached to both element declarations and particles, and we need to explain what happens if the same kind of constraint is specified both on a global element declaration G and on a reference to G.

For example,

<xs:schema>
  <xs:element name="global" targetType="type1">
  <xs:complexType name="someType">
    <xs:sequence>
      <xs:element ref="global" targetType="type2"/>
    </xs:sequence>
  </xs:complextype>
</xs:schema>