Billing Modifiers for Ambulance HCPCs Codes

Is there a stable, continuous good source for the single-letter "destination" billing modifiers associated with Ambulance claims? A section of Wikipedia, or some other site??

After having a break from these type of claims, I forget whether an "H" stands for "Home" or "Hospital," and the significance of the ordering of the two-letter combinations, too. For example: does "HR" definitely translate to, "patient transported from Hospital to Rehab. Center?" Or, should it instead equate to, "transport from Rehab. to Hospital"??