A quick example to hear how hold values slightly affect the sound (as opposed to just holding the note length).
Clear a track, load the SID engine, set the wave to mix and all other parameters to off/zero. Set the amp ATK and REL to zero. Set the hold to 6 and the decay to 24.
Tap a pad to hear the sound which should have a buzz to it. Now if you set the hold back to zero you should find that you can’t achieve that exact buzz no matter where you set the decay.
Don’t own an analog synth and never have (though I have played a couple). I recognise DSI but am not familiar with the other synths you have named.