What exactly is considered "Ketores Zarah"?
Rashi #1: Any Ketores that is donated (anything outside the prescribed daily amount that comprises the Ketores Tzibur) is considered Ketores Zarah.
Rashi #2 (in Zevachim, 27b): The Torah only permitted the Ketores shel Tzibur morning and afternoon.
Ramban (citing Targum Onkelos): It refers to Ketores that is made up of any combination of spices other than those prescribed by the Torah in Ki Sisa, 1 or even if one adds spices to those that the Torah prescribes.
Menachos, 50b: "Lo Sa'alu alav Ketores Zarah"
As laid out in Pitum ha'Ketores.