Although this panel does not include St Catherine, each of the following images originally formed part of the predella, which was added when Catherine was canonized in 1460.
Originally made up of ten panels, their combined width leads van Os (Sienese Altarpieces vol 2 p122ff.) to argue that there would have been large panels flanking this central image, one of which would have represented St. Catherine of Siena.
In April 1447, the rectors of the guild of grocers (Pizzicaiuoli) commissioned Giovanni di Paolo to paint "an altarpiece for the altar of the newly constructed chapel in the church of the Hospital of Sta Maria della Scala of Siena..."
A biography was written by her confessor, Raymond of Capua, and this work provided the source for Giovanni di Paolo's scenes below.

St Catherine kneels before an altar and reaches up to choose from the monastic garments offered by Saints Dominic, Augustine, and Francis, all founders of religious orders. Catherine takes the habit of Saint Dominic, which she wore as the founder of the Sisters of Penance. She also receives her girdle and her lilies.

The beggar was really Christ in disguise, and at the left he returns the cloak to her. For this act of charity, the cloak perpetually protected its wearer from the cold.

St Catherine miraculously receives communion from Christ while her confessor, officiating at another chapel in the church, raises his hand in astonishment at the consecrated wafer, from which a piece has disappeared.

St Catherine is mystically united with Christ in the presence of His mother. Music is provided by the King David.

St Catherine is shown elevated in rapture, holding her heart, which leapt out of her body to be united with Christ's.

Contemporary accounts of her life tell that on occasion St Catherine would be overcome by rapture, whereupon she was able to explain points of holy doctrine. Her pronouncements, which were dictated to a series of secretaries, were compiled into a book titled The Dialogue of Divine Providence.
Because Catherine is depicted with the halo of a canonised saint, it is likely that this panel was not completed until after her canonisation in 1461.