What is the difeeence between

X_min_home_pos = -10

Vs setting it at 0 and doing

Manual_x_home_pos to -10

According to the marlin docs one overrides the other.

Would both not result in a home position 10mm from the left end stop?

if your motors and endstops are configured correctly then a - value would move the axis towards the endstop since the firmware sees the endstop as position 0. A positive value moves away and a negative moves towards. If you are at the endstop then you will just hit the stop with a negative value and grind the motor till it times out or you kill power. Hopefully your firmware is setup so you can't do a negative once a stop is triggered.

As to your question, the firmware sees the first command and executes it by moving to home 0 and then trying to move another -10. However again for safety if your firmware is setup right it should not allow movement beyond a hard endstop or a soft endstop which is a limit set in the firmware based on steps rather than an actual switch.

The second command is one issued when you do a manual move in the host to home that axis. It will do what the firmware would but only when you execute it. Otherwise the firmware will do the first one on power up.

Again though for safety and IF your firmware and endstops are configured correctly it should not allow movement in a negative direction once a hard or soft endstop is reached.

The only time you would be able to move in a negative direction is if your endstops are actually configured as MAX enstops by being plugged into the max ports on the controller.. Then you could move away from them BUT you would not be able to move in a positive direction beyond them.

X_min_home_pos = -10

Vs setting it at 0 and doing

Manual_x_home_pos to -10

According to the marlin docs one overrides the other.

Would both not result in a home position 10mm from the left end stop?

As already mentioned - "home" designation varies depending on how the firmware is set up and where the endstops are plugged in on the board.
the endstop plugins and the firmware setup need to match for things to function correctly.

Hi, I starting to delve into the more technical side of 3d printer from the out of the box solution.  Just wrapping up a ramps conversion of my davinci which is almost complete but like to understand everything I do.

When people refer to home what is the position relative to the endstops?

For the Z I am assuming it is the endstop + offset which will position the head just above the bed.  What about the x and y?  Is the endstop 0,0 or is the centre of the bed 0,0?  How does one configure here to start the print or centre of bed.  Wouldn't it make more sense to have an offset set for the x and y similar to the z?

Offset depends on the movement of the printer. You have to keep in mind that Gcode which is what actually makes the movement was based on CNC programming and that relates to the movement of the workhead on the machine. So on some printers while the bed is what moves up and down and the head stays at one set height as far as offset and gcode is concerned it is still the head that is moving up and down. So it is actually the opposite for z offset on this machines.

To get the bed closer to the head it would be a negative offset. Picture the head moving instead of the bed even though it is the bed that is moving. On CNC machines only the head moves and that is reference that gcode uses to run the machine from. If you printer has a bed that is a fixed Z and the head moves up and down on a gantry then yes the offset would then be a positive value.

Another issue is what port are your endstops connected to on the board. If min then the home values would be 0,0,0 plus some offset set in the firmware. If they are in max ports then your home would be whatever the firmware has set for the max on those axis or the physical limit of travel on that axis. Some printers like the Solidoodle use MAX for home instead of MIN if my memory serves correctly.

As for centering you sometimes have to fudge the dimensions of the bed in your slicer. The bed center is resolved based on the actual home positions and not 0,0. The bed center is actually going to be a value between from X0 to MAX X bed size and the Y will a value between Y0 and MAX Y bed size. Usually the home for each axis is actually off the bed so you have to add that distance to the bed to get the center start position set right for the printer.

Think this is fun for one head try setting up a dual head.

However, you don't really need to adjust x and y offset very often unless you changed to different size of print bed. Or if you have two hotend, you need offset adjustment.

z-offset however may need more adjustment because if you level the bed or add a piece of glass or other print surface, you need to adjust your z-offset.

When people refer to home what is the position relative to the endstops?

For the Z I am assuming it is the endstop + offset which will position the head just above the bed.  What about the x and y?  Is the endstop 0,0 or is the centre of the bed 0,0?  How does one configure here to start the print or centre of bed.  Wouldn't it make more sense to have an offset set for the x and y similar to the z?