Umpire's judgement on whether they feel an injury requires immediate attention. Have to be there to make that judgement, in league I'm probably going to be more likely to call time immediately than I am at a National.An umpire shall not call time when any play is in progress, including when a thrown ball hits an umpire.
Exception: During live ball situations, when a player becomes injured and and requires immediate attention, an umpire shall call time.