There is no standard anymore. Maybe back in the day, but now you've just got a bunch of different apparatus named by tradition more than anything else. Seriously, there is no straightforward answer, but if you want me to get into it a little bit, you may be sorry you asked!
Generally speaking, "EQ" means "Equipment truck", but what exactly that means varies by dept. Most rescue trucks (heavy rescue, extrication equipment, etc.) used to be called "EQ" before the name "Rescue" became popular 20 or so years ago. Some depts still use "EQ" to refer to their rescue truck, while others use "EQ" to refer to a small equipment truck, not necessarily a rescue.
2-EQ = Ford F-550 "light rescue", basically a utility truck
3-EQ = Similar to 2-EQ
6-EQ = A Ford F-450 with a pickup bed and some compartment, used to haul around pumpout equipment and to tow the Tech Rescue trailer
7-EQ = Another "light rescue"
8-EQ = Nanuet's first due rescue-pumper from their main firehouse, also carries extrication equipment
9-EQ = A pickup truck used for brush fires and hauling stuff around
11-EQ = A heavy rescue large enough that it needs its own zip code
12-EQ = A heavy rescue
17-EQ = Their old ambulance from when they ran BLS prior to the late 90s, went by a variety of names over the years including 17-EM, 17-EMR, 17-Squad
18- EQ = Rescue truck
19-EQ = Rescue truck
21-EQ = Rescue truck
22-EQ = Rescue truck
23-EQ = Ford F-550 utility truck
24-EQ = Pickup truck
"EM" means "Emergency truck", but again the definition varies by dept. In some depts it is or was an ambulance, in other depts it was a rescue truck.
4-EM = Rescue with a high pressure pump
13-EM = BLS ambulance
As noted above, 17-EM used to be Spring Valley's BLS ambulance. 10-EM was a rescue truck, the current version is called 10-Rescue
There were other "EM" trucks in Rockland's history, most of them were equipment trucks that carried rescue equipment.
And before anyone makes a remark...no, there are no actual "tankers" in Rockland ("tenders" for you west coast folks). It's a traditional name from the days when carrying 1000 gallons of water was a big deal. No pumper/engine in Rockland carries more than 1250 gallons of water, nor do they have dump valves or any other such "tanker" features.
In the end, they are all just names. Everyone knows what apparatus the other depts have, so it's not a big deal that there's a variety. No one is going to mistakenly call 13-EM if they need a rescue truck, and likewise no one is going to call 4-EM if they need an ambulance.
