Weird Milepost Signs on NY highways

[TMETSJETSYT]

Ok, so I know that the milepost signs on state highways that aren't signed in NY have the weird 3di numbers, and thats not what I am focusing on here. What I am confused about is the numbers on the signs. I don't know what they mean. I wanna say that they are just simply mileposts, but the numbers look weird to be miles. Ill post links down below, but I have been confused about this for years and I just wanna know why. Also why do all the milepost signs on interstates have the I after the number, such as 87I, or 90I. Also, just wondering, what do the numbers mean on the 3di unsigned roads? Ill post links down below.

Weird Milepost sign
https://www.google.com/maps/@42.6023333,-73.8106013,3a,15y,94.24h,88.05t/data=!3m6!1e1!3m4!1slHSFm89iCTINKtay2jIs_g!2e0!7i16384!8i8192?authuser=1

https://www.google.com/maps/@42.1148026,-73.7152407,3a,15y,206.4h,86.31t/data=!3m6!1e1!3m4!1smArnvZpkPwotJb3tFBeSTw!2e0!7i16384!8i8192?authuser=1   (This is on the Taconic State Pkwy and there is a milepost clarifying that it is 79 miles, and none of the numbers on the 987G post match that, so it can't be miles.)

[Mapmikey]

Explanation from the source - https://www.dot.ny.gov/divisions/engineering/design/dqab/rmm

[empirestate]

This is my department: http://www.empirestateroads.com/rm/

[TMETSJETSYT]

Thanks guys. That explained it all.

[US 89]

My eyes hurt from all those smileys.

[ixnay]

I always liked those little NYS markers.  I look for them whenever I'm driving/riding in the Empire State.  They even have them on urban expressways.