Halliday & Matthiessen (1999: 400n):
Both these systems, THEME IDENTIFICATION and THEME PREDICATION, assign exclusive identity to the identified term of the equation. They differ, however, in the kind of prominence this identity carries with it:in THEME IDENTIFICATION, in which the original figure is restructured, the effect is one of second-order (semiotic) prominence, whereas
in THEME PREDICATION, where the figure is not restructured, but one element within it is explicitly predicated, the effect is that of assigning first-order (natural or inter-subjective) prominence to the predicated element.