A T1 is requested by the company, usually because it is about to release something material such as a merger, a clinical result, or a restatement. Trading stops on every venue, not just the primary listing exchange, which distinguishes a regulatory halt from a venue-specific pause.
Resumption follows a fixed sequence: a quotation-only period where orders can be entered and cancelled but not executed, then a reopening-auction. Positions cannot be exited during the halt, so the gap on resumption is unavoidable risk.
Example: a stock halts at $18.40 on a T1. The company announces an all-cash acquisition at $27. The reopening auction prints $26.10, and no trade was possible anywhere between $18.40 and $26.10.
Related: t12-halt, trading-halt, reopening-auction, limit-up-limit-down, merger