<Control Links> and <Expressions>
The fields of the <Control Link> specify
- which field(s) of the target are being modified
- for each target field being modified, which <Expression> component of the <Control Link> is supplying the target field value
The syntax makes it look as though a <Control Link> may use any <Expression> to do this - <Literal>, <Function>, or <Variable>.
The semantics are actually tighter than this. <Control Links> are used only when a <Variable> (or a <Function> with a <Variable> argument) is used as the controlling <Expression>.