Bottom line
Every day you serve in the Reserve falls into a system. First, why you are being used: training, support, mobilization, or other. Second, what kind of duty it is: inactive duty (ID) or active duty (AD). Third, which pay group you sit in, which is set by how much training you owe. This section defines those buckets. Nothing here is a task for you. It is the framework the rest of the manual builds on.
What it means for you
You do not pick your utilization category or your pay group. They follow from your unit, your status, and the orders you are on. But knowing them helps you read the rest of the policy. If you are SMCR, you are pay group "A" and you owe 48 IDT periods and 14 days AT a year. If you are an IMA, you are pay group "B" and you owe up to 48 IDT periods and at least 12 days AT a year. AR Marines are pay group "Q." Marines in initial training are pay group "F." Figures 3-1 (utilization authorities) and 3-2 (duty types) lay out the full picture.