intensity minutes and activity class

No, "Activity Class is a measurement of how frequently and how long a person trains in a given week.", so it doesn't have anything to do with intensity minutes.

https://support.garmin.com/en-US/?faq=DJEru6ns626MZTh2kvUXZA

Which sport would fall into category 1? It seems that in the lower categories ... 1 and 2 fall most of the people who do not play sports 

These are the activity classes recommended by firstbeat, and it seems that most people fall into category 1 ... even without doing sports 

The type of sport is irrelevant for activity classes. What is important is the how often you do it and for how long time.

This is how I read the chart:

Category 0-3 are for people that don't really do any training. 0-1 activities per week. The occasional walk or maybe a short run in the weekend.

Category 4-6 are for "normal" people that go on an occasional run a couple of times per week or hit the gym.

Category 7 and up are for people that exercise often and regularly. You will know which activity class to pick since you log all your training and know exactly how often and for how long you train.

Just check your training calendar and from that pick the activity class.

ok ... what's the point of telling my garmin how much activity I do?  

I have read Affect Calorie Calculation ... How? 

7518047 said:

I have read Affect Calorie Calculation ... How? 

It has impact on the EPOC (excess post-exercise oxygen consumption) and hence adding up Calories. Read more on that topic for example in the whitepaper from Firstbeat (the provider of the algorithms) [EPOC Based Training Effect Assessment]