I am trying to understand the relationship between Marlin firmware ( Ramps 1.4 ) & Simplify3D. Marlin ( firmware ) you add the build plate dimensions in and then when you setup your Simplify 3D you add the same build dimensions into Simplify3D. Basically you adding the same info into both areas.
So which setting does Simplify 3D use Firmware or Simplify3D settings ?
Does Simplify3D reflect / show the Eeprom settings and can you change your eeprom settings in Simplify3D ?
Firmware dimensions are set software limits for max movement area. Slicer dimensions are the limits for print area. They don’t necessarily have to be the exact same, depending on the printer.
My printer have the zero position off the build area for purging, so if the slicer read the eeprom setting you could end up printing in the air.
Ok , so what you saying is that the Firmware is there to prevent the head moving too far out of area and into something. So you can have firmware 200 x 200 but build area is 180 x 180 which you have in your slicer ( Simplify3D ) ?
Exactly. 
You would need the option to set zero position to a negative value though. In slic3r (and possibly other slicers?) you can also make a custom build area based on a svg vector drawing, to avoid hitting mounting clips or use a hexagon bed for instance.
Most of the time the firmware and slicer platform size could be the same, and you would manually make sure you don’t print outside if the area.
One time I printed a model similar to a upside-down cone, and while the bottom fit inside the print bed, the top part was outside, but that was a extreme case. Worked find though since I didn’t have any limits sets.
The way you want to look at it is like this:
Firmware sets your travel max and min values, but your slicer also needs the correct dimensions. It uses those dimensions to plot x and y coordinates. For example, if you have an 200x200mm build plate and you configure it correctly in your firmware, but leave it as 150x150mm in your slicer, the slicer is now going to send the hotend to G0 X75 Y75 Z0 if the object is positioned in the center of the bed in the slicer. Basically the slicer uses your input values to decide where center points as well as other points are on the bed. Configured correctly, it would move to G0 X100 Y100 Z0 to get to the center.
I getting it now, thanks guys.