system definitions

Hey all, I know you can set your hackintosh system definitions to whatever via multibeast, but I'm not sure in what ways it significantly affects your Hackintosh. I have a Skylake desktop which I think is set to the default iMac definition, but maybe I should be setting it to something else...

I was trying to update from Mountain Lion to Sierra. When I was trying to download Sierra from the App Store to create the boot USB, it wouldn't let me. So, I used these instructions to change the System Definitions to iMac14,2...

I would be glad to know, what is the purpose of the system definitions and how we choose between one of them... Is there some rule when selecting a definition based on the hardware (and software - OS X) ? Or this is just a visual thing and nothing more ? Thanks in advance.

I'm following Alfa's Gigabyte UP5-TH guide, and I had a question about the system definitions - what's the difference in choosing them? I originally tried MacPro 5,1, but received kernel panics. Leaving the definitions alone gave me MacPro 3,1, but the guide says to use iMac 12,1. (Why not...