Q. 1. What is the Biretta used for in the Catholic Church?

A. 1. The biretta is a stiff square-shapped hat with silk trim and tuft. It has three or four ridges, called "horns," across the crown. It is worn by the clergy. It is black for priests, deacons, and seminarians, purple for bishops, and scarlet for cardinals. The biretta is now optional for clerics who are celebrating or concelebrating Mass.

The biretta originates from the Latin words, "biretum" and "birretum."

Quoting "https://en.wikipedia.org/wiki/Biretta",

"The biretta may be used by all ranks of the Latin clergy cardinals and other bishops to priests, deacons, and even seminarians (who are not necessarily clergy). Those worn by cardinals are scarlet red and made of silk. After the Second Vatican Council the ceremony of giving the galero to cardinals was replaced with giving the biretta. The biretta of a bishop is amaranth in color, while those worn by priests, deacons, and seminarians are black. The pope does not make use of the biretta."