If this helps in any way, the only reference to 'Thr Des' I can find from my books on th A320, is from the FCTM Normal Procedures under descent where it says:
"The idle segment assumes a given managed speed flown with idle thrust plus a small amount of thrust. This gives some flexibility to keep the aircraft on the descent path if engine anti-ice is used or if winds vary. This explains THR DES on the FMA."
Never seen it in real flight before though.