I’ll answer what I can for you.
Yes leave unassembled all the unused, just make sure all capacitors, diodes, resistors, and both 7805 voltage regulators and LED are assembled.
R8 is a wire. If you assemble resistors 1st, you will have the end off of one for the R8 jumper.
On-off is just a switch to close that circuit to energize the big relay to power the pcb.
Steer is optional. Button for momentary contact to engage autosteer. Can use touch screen instead.
Remote is for disengage if the steering wheel is turned.
Only use com and 0 on switches. Should watch some of the assembly videos, if you haven’t assembled before.
I recommend using K4 (relay), but if you don’t, you need to supply power to one of the 12v Out pins.
The 12 V Out pins are for powering the MD13S, 12 V to 24 V converter … and so on.
BTS442E2 is needed, if you want to use Sw Out 12V pins.
Sw Out +12v pin can be used for 6/2-valve (optional) to connect hydraulic steering valve lines to the steering cylinder when autosteering is enabled. Sw Out +12V pin provides +12volts when autosteering is enabled.
I’m not sure, but Sw Out +12v might also be used to supply +12V to MD13S to prevent it’s braking feature when powered but not having any pwm signal coming in. Could someone correct me if I’m wrong?
SJ1 jumper needs to be soldered regarding which motor controlled used.
If you want to use Sw Out 12V as a normal 12V output, you can leave BTS442E2 out and add a thick jumper wire between the pins shown on photo below.
Steer is for autosteer switch/button.
Imp is for workswitch.
Remote is for turn sensor/pressure sensor.
Analog in/CAN/Aux A/D connectors are there for just in case. I would like to hear if someone finds a use for them 
You can leave the R8 empty if you use a simple 2-pole on-off switch on the Streer, Imp and Remote.
EDIT: Corrected wrong part name
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Thank you for your explanation.
This PCB is awesome.
I have now used a wire jumper instead of a resistor on the R8. Is this ok?
I would like to use the SW Out function. Unfortunately I can not find a source to buy the BTS422E2. Do you have a recommendation for this?
I also have a question about the connection of the MD13S. Would this connection to the board be correct?
PCB __ MD13S
GND → GND
DIR1 → DIR
PWM1 → PWM
PWM2 at MD13S and NC at MD13S not connected
Is the bts432e2 maybe an option?
EDIT 5.12.2021 Corrected the wrong part number
Thank you.
Wire jumper at R8 is ok.
BTS442E2 can be found from AliExpress. There are couple links to AliE in PartsList_KaupoiMODv4_1.xlsx at KaupoiMOD v4 folder. Click the url at my first post.
—MD13S—
-----PCB-----
Thanks for your help.
At the PCB and Wiring Diagramm the Part is labeled with “BTS422E2” but in the PartList I onely find the “BTS442E2”. Is the BTS442E2 the right one?
I hope not, I just ordered some 422’s from china
Do i need to insall a jumper at the SJ1 If im using an MD13s without the BTS422E2?
Oh no! PCB has fault info on it. The correct part is BTS442E2 as told in PartsList and in the url’s in it. 
Thanks for finding it.
@Jhmach You sure you ordered 422? …and sorry if you did… I just can’t find such part anywhere with google.
@kansasfarmer No need to solder jumper if not using BTS.
That’s what I get for not looking at the BOM.
Got some coming off Ebay. Thanks Kaupoi
pwm2 is connected to ibt2 in SJ1, from what it looks like in the schematic, correct ?
If using BTS442 and IBT2, solder SJ1 pins 2 and 3 together.
If using BTS442 and MD13S, solder SJ1 pins 2 and 1 together.
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I have been using an opto relay to power my valve, and pwm2 energizes it. So does pin 3 connect to pwm2 ?
SJ1 pin1 is connected to PWM2.
SJ1 pin3 is connected to DIR.
Oh how I would have loved that connector today soldering the jumper wires to my V2 PCB 
Going to run the steer valve on CAN instead of Cytron.
does the BTS442E2 have enough power to run a 12 volt valve directly. or do i need to have a relay between it and the solonoid?
If I understood the datasheet correctly, it should be able to handle 21 amps. So the control of one valve solenoid is a piece of cake for it.
See datasheet on my post above.
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Did some testing today. With autosteering off, when +12V power supply to MD13S is not connected, but GND is, MD13S is braking about 50% force (by fingertip feeling) comparing if it has +12V ang GND supply connected. Directional led and ERR led are also on if turning motor by hand when +12V not connected.
I also tested how much the BTS442 heats up if the motor is braked to a standstill. I held motor by hand and did not let it spin at all and BTS442 got really hot in few seconds. I think it’s good to have a heatsink on it.
It is not understandable to me why we use the BTS422E2 chip, which was produced in the 90s.
Ordering this system from China is almost a miracle for it to work properly. Not to mention its price.