Trading in a stock can stop for reasons that have nothing in common with each other. The mechanisms are separate systems with separate triggers and separate durations.
News pending halts
A listing exchange can halt a stock at the issuer's request or on its own initiative when material news is imminent, giving the market time to receive and process it.
The purpose is to prevent trading on unequally distributed information during the moments around a release. The halt typically ends after a set dissemination period.
Reopening runs through an auction process that collects orders before resuming continuous trading, which is why the first print after a news halt can differ sharply from the last one before it.
Volatility pauses on individual stocks
A separate mechanism pauses a single stock when its price moves outside a band calculated from recent trading, without any human judgment about the cause.
The bands are percentage-based, wider for lower priced securities and wider near the open and close when volatility is naturally higher.
These pauses are short. Their purpose is to interrupt a runaway sequence long enough for orders to arrive on the opposite side rather than to investigate anything.
Market-wide circuit breakers
A third mechanism halts all trading when a broad market index falls by defined thresholds from the previous close, with successive levels producing longer stops.
The first two levels pause trading for a set period, while the deepest level closes markets for the remainder of the session.
These thresholds were revised after episodes where the original design proved either too loose to matter or calibrated to a market that no longer existed.
Regulatory halts and their consequences
A securities regulator can suspend trading in a security, typically over concerns about the accuracy of public information about the issuer.
These suspensions last considerably longer than exchange halts and can leave a security unable to resume quoting on an exchange without meeting specific requirements afterward.
Resumption often occurs in a different trading environment than before, because market makers must satisfy conditions before publishing quotes again.
Why the type of halt matters to orders
Orders resting in the book behave differently across mechanisms, and orders entered during a pause participate in the reopening auction rather than executing immediately.
Because reopenings are auctions, a market order entered during a halt executes at whatever price the auction produces, which may be far from the pre-halt level.
Understanding which mechanism is in effect tells a participant whether trading resumes in minutes, at the next session, or only after a regulatory process concludes.