Differences between loot distribution files

[Nagyx]

Soo... I'm new to modding, and so far so good with item creations, recipes, and easy stuff.
But now came the hardest part for me... The loot distribution.
I know I know, many of you guys might say to just look into mods and try to make it similar, and yes it worked for me in so many cases.
But let's go to my QUESTIONS: 
1) The loot system has changed not long time ago right? What was the obsolete system?

2) What's the difference between Distribution.lua; ProceduralDistributions.lua and SuburbsDistributions.lua?
3) When creating a mod, which one of these guys above should I use? 

PS: Not a native english speaker. So, be kind with my typos =D