Q: For normal usage, install D1-D4. There will be NO SW1 …
SW1 allows you to switch between the clipping options that are On-board. Only when using A/B pads for off-board modding you do you not need SW1 since you are already going to be switching using a switch which is Off-board.
Even if you do not have any experience reading a schematic you should always practice looking at them since in this case you will see D1 through D4 and you will see the switch symbol showing how it switches. It can become quite intuitive very quickly. In the image below it shows you that there are two sides to the switch SW1a & SW1b (see components guide) so the schematic shows you are switching between D1 through D4 on SW1a and D5 & D6 and SW1b for your two different on-board clipping options.
I hope that helps.