The clarinet has a unique dynamic spectrum in that its middle register is softer than both the lower and upper register. It has a wide range, with solo repertoire extending an octave above the printed range above.
The lower register is very dark, and is effectively used in mysterious passages. The brightness of the upper register is often avoided in traditional orchestration, but it can be used to great comedic effect. This should not discourage one from writing in the upper register as it can be quite beautiful with a good musician, but the average church musician will sound very bright above the staff. Therefore, it is common for the clarinet to double the flute at an octave in church orchestration note-for-note.
The B-flat Clarinet is a transposing instrument. Though other clarinets exist, it is rare for the modern composer to write for these instruments - especially in a church music context. Therefore, care must be taken to write the correct pitch when arranging for clarinets.
The Bass Clarinet is also pitched in B-flat, and sounds a 9th lower than the written pitch. Parts are written in treble clef, not the bass clef. It can double the bassoon, euphonium, or cello. It has the same dynamic spectrum as its higher cousin. Therefore, the bottom register can be quite powerful. The upper register isn't used extensively due to its bright timbre.