The first hour, traditionally the first two half-hour TPO brackets, sets the day's opening boundaries. A session that stays inside it is balanced; one that extends beyond it is said to show range extension.
The useful classification comes from how much extension occurs. A wide initial balance with little extension suggests a rotational day where fading the edges is viable; a narrow one broken early suggests a trend day where fading is dangerous.
The one-hour convention is a convention, not a law, and it derives from pit-session structure. In markets that trade around the clock, defining when the session starts is an arbitrary choice that changes every conclusion drawn from it.
Related: market-profile, opening-range, balance-area, prior-day-high-low, session-breaks