Solo and accompaniment registration is used in many hymn preludes:
1. there are TWO MANUAL PARTS OR MORE (see purple and yellow highlights)
2. ONE PART IS PROMINENT or more melodic in nature (purple highlight).
The pedal may or may not be present.
This differs from chorus registration (see green highlight), in
which:
1. there are two manual parts or more (as in solo and accompaniment),
but--
2. NO PART IS PROMINENT or more melodic in nature.