In have no idea what the abbreviation stands for nor its interpretation for the singer. The positioning of this abbreviation sits atop the staff sometimes. I could only find the (multiple) abbreviations for "SA" (meaning) but not how to sing this.
In a four-part chorale texture, there are four voices singing: soprano (the highest), alto, tenor, and bass (the lowest). We commonly call this an SATB texture, although there are other arrangements.
The "SA" stands for "Soprano and Alto," and it indicates which voices are shown on a given staff.
In the example below, "SA" next to the treble clef indicates that the voices in that staff are the soprano and alto; the "TB" next to the bass clef indicates tenor and bass.