Mostly, those are recommendations found in company safety manuals, AISC General Safety Guide for Fabricators, AWS Fabrication Safety, ANSI Z-49 (also available from AWS) and many other guides.  Most of these can be found for downloads but may not be exact titles.

Many other of the AWS Welding Handbooks and other resource welding instruction books will have guides/charts that are very similar in their recommendations.  They are usually based upon amperage, fuel pressure-tip size, welding/cutting process, and other factors that basically come down to illumination brightness factors to determine a safe exposure level for the human eye. 

Of course, older people want to see clearer with failing eyesight so will use less darkness shades of lens.  Inside a building with dim lights as opposed to outside in the sunshine.  Type of lens such as auto dark so you can see better without the arc struck and then not go blind when the arc is initiated.  Many other factors will help determine what shade works.

A company should establish a basic minimum for various processes for liability protection without making it overly restrictive for the production crew to choose what works for them. 

Directly to your question, yes, type of gas will make a slight difference because of the different ways they burn and the reactive effect when the flame is applied to the work. 

Personally, a #3 is about the lowest you really want to ever be for torch/brazing work.  For most people, probably a #7is would be the highest ever needed with #5 being a good average for many oxy-fuel applications both in welding and cutting. 

He Is In Control, Have a Great Day,  Brent