Retirement tax questions

Great, what is the 'plain' 1099-R?  They are named by TT as "Financial Institution or other provider", "Office of Personnel Management", "Office of Personnel Management", and "U.S. Railroad Retirement Board".  When you say "plain", do you actually mean "The one marked most common, Financial Institution or other provider"?  It's amazing that TT doesn't make this more clear given the sheer number of folks who are retired military in this country.