I am sure you know that ac means before meals. Reason for this is to determine blood sugar before eating to see if there are insulin needs not being met by natural insulin, added insulin or oral meds that increase insulin production. That's the basics.
Biggest reason for hs check is to be sure the person is not likely to have an insulin reaction in sleep. This is very scary and many people have died as a result of lowered blood sugars.
When people are under any type of stress-infection,illness, emotional, lack of sleep, certain meds., etc. They will have higher blood sugars. We usually have to titrate insulin to the individual requirements as their conditions change.