The "generator overwrites" tickbox is a just a choice that the software puts up to the user (you).
If you tick this box and the click on a channel, then the s/w will copy the currently shown settings in preset, program and generator setting tabs into a specific area of the computer's memory related to that channel.
If you do not tick the box, then it will leave the memory as it was. (and... the preset, program and generator settings tabs will revert to what was stored previously for that channel)
Once you press start in the channel box; Spooky will use what is in memory and transfer the settings, waveforms etc. to the related XM generator.
So, ticking the "generator overwrite" box and then selecting the channel could be translated to mean- "do you want to use the new settings with this generator".
If you click on a generator you will either want to:
1. Open it up to see what is currently loaded (do not check overwrite).
2. Upload a new program to the generator (overwrite the current program, check overwrite).
For more details, please check the link: