Both of my Ensoniq synthesisers (ESQ 1, SD 1) had key zones available to all available parts. Each part could play internal, external, or both voices. Each part could be enabled / muted on the fly using single and double button clicks. There were 12 parts available on the SD 1 as two banks of 6. Used a highly readable two row display. This is a high level of flexibility with instant access and I really miss this feature.