Bluelab PPM Pen

Ok so I need someone to explain to me the different features this pen has. It gives you a choice of using a 500 ppm or 700 ppm reading. Which scale should I use? When I use the 500 scale for example I get a reading of 192. When I use the 700 scale it gives me a reading of 296. So which one should I go by?

I can use 700 ppm, or EC, or 500 ppm as measuring scales thanks to the compatibility of combo pen by Bluelab, as for your question I can't answer why, but the guy who sold me this told me to use by 700ppm scale. Oh if you have a ph pen as well remember to recalibrate it, unless you use pH perfect nutrients, as for pH perfect technology, I haven't really touch the ph down since I used advanced 3part ph perfect. Since my pen always stays @ 6.0 after topping nutrients and water. I use the RE-DWC method.

i to have bought this pen, from what i read most people use 700. I read that ec is the original measurement then both 500 and 700 are different conversions of that.
I was wondering which scale to use but then realised you will be altering your ppm with your water anyways ie if your plant is drinking water but the ppm is staying the same you have the correct amount of nutes
hope that makes sense and people feel free to correct me if im wrong

USA: 500
EU: 640
Australia: 700

Most nutrients tell you what scale they use or they use ec or cf