RTH is a convention inherited from the floor: for US index futures it is 9:30am to 4:15pm Eastern, matching the cash equity day. Charts can be drawn on RTH data only or on the full 23-hour session, and the two produce different highs, lows, gaps and vwap anchors.
Traders should pick one and know which one their levels came from. An RTH-only chart shows a gap where the overnight session traded; a full-session chart shows none.
Example: if ES trades down to 4,980 at 3am and RTH opens at 5,010, an RTH chart's low for the day may be 5,005 while the full-session low is 4,980. Stops placed from the wrong chart sit in the wrong place.
Related: eth, overnight-session, globex, extended-hours, opening-range