Many modern vehicles have trip odometers, a vehicles odometers can be set to record Miles or Kilometers. You will have to read your vehicles manual to learn how to make changes to these settings.
A - is for Trip odometer A.
B - is for Trip odometer B.
M - is for Miles.
Km - is for Kilometers.
There is the main odometer that records the Kilometers (Km.) the car has traveled since the day it was new from the factory. Then there is one or two trip odometers labeled A and B.
Let say you Wanted to drive from Agra to Jodhpur but you wanted to stop in Jaipur and maybe even go see your cousin in Kishangarh and when you get to Jodhpur you want to know how many Km. you drove on that trip. You could write down the main odometers reading before you start the trip and then record the main odometers reading when you arrive, then subtract the first number from the second number and this will tell you the kilometers traveled on your trip.
BUT there is an easier way because you also have trip odometers, You set the trip odometer A to 0 (Zero) before your trip and when you arrive it will tell you how many Km. you have driven since you set it to 0. You can reset it if you wish before your return to Agra, or not.
Maybe when you get to your cousins place in Kishangarh he wants to go for a drive to Nasirabad and you want to know how many Km. you have traveled from Kishangarh to Nasirabad and back to Kishangarh. You set trip odometer B to 0 (zero) in Kishangarh and when you get back to Kishangarh from Nasirabad it will tell you the amount of Km. that trip took. This will also add those same Km. to trip odometer A, but since you know how many Km. that trip took you can subtract that number from trip odometer A so that the Nasirabad trip Km. is not factored into the trip from Agra to Jodhpur via Jaipur and Kishengarh.
OR maybe you want to know how many Km. you travel in a specific time period. You can set trip odometer B on the first of the month and check it on the last day of the month see how many Km. you traveled in that month.
Are you thoroughly confused? The bottom line is; You have three odometers in your car, The main one that you can not alter or change and two trip odomets that you can reset anytime you like. You can reset trip odometers A and B to 0 any time you want and it will not affect the main odometer that records the total Km. the car has traveled since it was new.