Forest Road Naming Convention

So I know I’ve seen an explanation to this in the past but can’t for the life of me find it. Forest Roads that are written vertically within the red box (on a USFS map) are designated for high clearance vehicles. But I know there is additional meaning to it if there are 4 digits and another meaning if a letter is appended to the end. Attached is a random picture of the USFS map near of Sedona. You can see some roads are three numbers, some four, some with a letter. Anyone know the answer? Bonus points for citing your source!

 

And here I thought the most recently designated road has the highest number.

 

Of interest:

https://wazeopedia.waze.com/wiki/US...ally recursive based,stub B, secondary stub 1.

https://0201.nccdn.net/4_2/000/000/081/4ce/Forest-Service-Roads-Glossary.pdf

 

USFS Naming Conventions

These naming conventions may help you understand the online GIS maps and PDF maps available from USFS.

Naming is typically recursive based upon road stubs. For instance, if a main road has name "33" then stubs from that road may be, 33A, 33B, 33B1, 33B2, 33C, etc. using alternating designations (letters, numbers). The third level of “33B1” is road 33, stub B, secondary stub 1. The exact choice is of this format is picked by different road engineers. See the image to the right for a pictorial of this concept.

https://wazeopedia.waze.com/wiki/USA/Forest_Service_Roads

If you zoom out in your provided map, maybe you can see the base roads.

 

Good find on the stub routes. Still not seeing anything on number of digits yet.