The power system has three jobs in sequence. Generation makes the power. Transmission moves it long distances at high voltage. Distribution carries it the last stretch to homes and businesses.
The distribution grid starts at the distribution substation. There, a power transformer steps the transmission voltage down to a level the local lines can use. From the substation, feeders branch out along streets. Smaller transformers near each group of customers step the voltage down again, this time to the level that runs lights and motors.
When you size equipment or read a procurement spec, you are almost always working somewhere on this last stretch. Knowing where a piece of gear sits between the substation and the meter tells you most of what you need to ask about it.