Comment by analog31

The above post mentions big band on double bass, which is precisely my main bag. This is just to clarify. The sheet music consists of portions that are fully notated (i.e., dots on the staff) and lengthy sections with nothing but chord symbols. I'm expected to improvise those parts, and to take liberties with the notated parts as well.

So it is indeed a mixture of reading and improvisation. But this is aside from the choice of notation, which is just a practical matter of finding people who can read it fluently enough to perform from it.