A T12 is not routine. It typically means the exchange wants explanations about unusual price activity, unverified claims, or missing filings, and it can last for days rather than minutes. It frequently precedes a deficiency-notice or, on the over-the-counter market, a longer regulatory suspension.
Reopening after a T12 is often violent in one direction, because the stock has had days of accumulated news with no way to trade and holders have had time to decide.
Example: a small cap runs 340% in three sessions on a press release. It is halted T12 for four days while the exchange seeks documentation. It reopens 71% below the halt price.
Related: t1-halt, trading-halt, deficiency-notice, delisting, pump-and-dump