From this Frequently Asked Questions pdf:
“The Limit Up-Limit Down (LULD) mechanism is intended to prevent trades in National Market System (NMS) securities from occurring outside of specified price bands. The bands would be set at a percentage level above and below the average reference price of the security over the immediately preceding five-minute period. The reference price will be the arithmetic mean price of eligible reported transactions over the past 5 minutes.“
“The Processors (UTP/SIAC) for each NMS stock shall calculate and disseminate to the public a Lower Price Band and an Upper Price Band.“
“The functionality … will apply beginning from 9:30 a.m. ET and end at 4:00 p.m. ET each trading day, except in cases of an early scheduled close.“
“Effectively, the band percentage changes based on the time of day and differs according to the price of the stocks.”