A company usually asks for a halt to release news that may materially effect the company. Once the news is out and everyone has had a chance to get the news at the same time, trading will resume. The news could be either good or not so good. This type of halt happens during the trading session. It isn't too common when news is released after hours, but on occasion a halt could happen after hours and extend into the next trading session. Also, if the OSC notices unusual trading they can halt the trading and ask the company if anything is pending or if they can explain why there is unusual trading. Someone else might be able to add to this.