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• Style 1: the tune has changes, and the soloist follows the changes and their chromatic alterations and makes sure that her actions are compatible. This is the traditional jazz way. Everybody has lots of things to keep track of. Comping has to follow harmonic changes, and the soloist has to follow harmonic changes and take into account the restrictions the changes impose on how she can twist the harmony. Air Traffic Control State of Mind! Lots of moving objects in the air that must not collide.
• Style 2: the tune has changes, but the soloist plays safe pentatonic notes around the tonic only and DOES NOT (HAVE TO) TRACK the tune's changes and their chromatic alterations. This is the blues/rock way. The soloist only has to react if the tune modulates so radically that the pentatonic around the tonic doesn't fit anymore.
• Style 3: the tune does not have changes or chromatic alterations. The soloist is free to do whatever she likes. Play safe pentatonic, or stick to the same scale as the tune has, or invent her own changes and chromatic alterations, maybe play pentatonic from other keys. Or anything. Complete freedom for the soloist. This is the modal jazz way. It's like style 2 but with the roles reversed and the whole situation turned upside-down. The tune is stripped of "changes", and comping plays a harmonically static groove, in order to set the soloist totally free.
• Style 4 could be a variant of style 3: the tune is modal, but the comping plays such thick and dense harmony that it in practice restricts the soloist's freedom. This can be a desired situation, or not. Had the comping played something harmonically simpler, it would have left more space for the soloist.

• Style 1: the tune has changes, and the soloist follows the changes and their chromatic alterations and makes sure that her actions are compatible. This is the traditional jazz way. Everybody has lots of things to keep track of. Comping has to follow harmonic changes, and the soloist has to follow harmonic changes and take into account the restrictions the changes impose on how she can twist the harmony. Air Traffic Control State of Mind! Lots of moving objects in the air that must not collide.
• Style 2: the tune has changes, but the soloist plays safe pentatonic notes around the tonic only and DOES NOT (HAVE TO) TRACK the tune's changes and their chromatic alterations. This is the blues/rock way. The soloist only has to react if the tune modulates so radically that the pentatonic around the tonic doesn't fit anymore.
• Style 3: the tune does not have changes or chromatic alterations. The soloist is free to do whatever she likes. Play safe pentatonic, or stick to the same scale as the tune has, or invent her own changes and chromatic alterations, maybe "go outside" and play pentatonic or harmony-outlining lines from other keys. Or anything. Complete freedom for the soloist. This is the modal jazz way. It's like style 2 but with the roles reversed and the whole situation turned upside-down. The tune is stripped of "changes", and comping plays a harmonically static groove, in order to set the soloist totally free.
• Style 4 could be a variant of style 3: the tune is modal, but the comping plays such thick and dense harmony that it in practice restricts the soloist's freedom. This can be a desired situation, or not. Had the comping played something harmonically simpler, it would have left more space for the soloist.
• (For completeness, I have to add that it's possible to go outside even on traditional non-modal tunes that have changes, two-five-ones and all that. IMO, playing outside means that the soloist builds a competing harmonic context which diverges from that of the backing instruments and the listener's expectations. The listener starts to recognize a different "drawing" being superimposed on top of the backing music. An extreme example of such soloing could be George Garzone's Triadic Chromatic Approach, which constantly moves the perspective around without letting the context settle down at all.)

Is there a process I can follow to determine what scale I can improvise with? Like with the C# diminished 7th

Can someone explain why lead sheets have non-diatonic chords

Is there a process I can follow to determine what scale I can improvise with? Like with the C# diminished 7th

Can someone explain why lead sheets have non-diatonic chords

The C note is released or pulled to the regular place by the chord's C note, but now the E note is pulled down i.e. made flat by the Eb note in the Cm6 chord. You might see that it looks like the Locrian scale, and why not. But to me it's much easier to think of it as a basic natural minor with one alteration. Talking about Locrian has a certain weirdness to it, because this is not a modal tune, and the chord is only played for two bars.

The C note is released or pulled to the regular place by the chord's C note, but now the E note is pulled down i.e. made flat by the Eb note in the Cm6 chord. You might see that it looks like a Locrian scale on the "6" note, or Dorian on the "5", and why not. Mentioning mode names has a certain weirdness to it, because this is not a modal tune, and the chord is only played for two bars.