The Harmoniemesse is the last Mass Haydn wrote. During his life, he wrote a total of fourteen masses. Just as the Heiligmesse I wrote about yesterday, this one was written for prince Nicolaas II Esterházy, his employer at the time. The name Harmoniemesse was given to it much later, and might refer to its elaborate use of wind instruments: these were used in what the Austrians called Harmoniemusik.
The Salve Regina that takes up the bigger part of side B of this release, was written a lot earlier, in 1771. Haydn was not yet the greatest (and wealthiest) composer in the world he would become in 1800, but he was already adventuring in harmonies, and showing his musical genius. The theory goes he wrote it to thank the blessed Virgin for his recovery from a disease that nearly cost him his life in 1770.