It is the custom of several religious orders to add a "title" to their religious names. For example, St. Thérèse of Lisieux was known in her community as "Sr. Thérèse of the Child Jesus and the Holy Face."
St. Thérèse's title reflected her personal devotion to the Child Jesus, and later she received permission to add "Holy Face" to her name, as she was also devoted to the Holy Face of Jesus.
For St. Faustina, she was given permission to add "of the Blessed Sacrament" to her religious name.
This name reflected her deep love of Jesus in the Eucharist, a love she possessed at an early age.
She describes this love in her Diary, explaining that she would spend any free time she had adoring Jesus in the Blessed Sacrament.
In 1935 she even received "the Blessed Sacrament" as her special "patron" for the year. Typically patron saints were given on New Year's Day, but for St. Faustina, Jesus in the Eucharist was her patron.
St. Faustina firmly knew that Jesus was present in the Blessed Sacrament, and since she loved him dearly, it was the primary place on earth where she wanted to be.
