Answer the following question: Given the following passage  "When not celebrating Mass but still serving a liturgical function, such as the semiannual Urbi et Orbi papal blessing, some Papal Masses and some events at Ecumenical Councils, cardinal deacons can be recognized by the dalmatics they would don with the simple white mitre (so called mitra simplex).",  answer the following question. Note that the answer is present within the text.  Question: Who are the main subjects of the passage?
Answer:
cardinal deacons