RTH means regular trading hours, the primary US cash-session window for stocks and ETFs. ETH means extended trading hours, a broader electronic window including evening, overnight, and premarket futures activity.
SPY and QQQ have regular cash-session hours, while ES and NQ remain available for most of the weekday outside a short maintenance break. This explains why an ETF can open with a gap.
Always label which session produced a high, low, close, or profile. Mixing RTH and ETH references can make accurate numbers look contradictory.