1) Pre-determined redshirt means that the coach/player go into the season with the thought or plan that the SA will redshirt. This means that if the player plays one point in a match, at anytime during the traditional season, then they have used one year of eligibility. A traditional redshirt tends to occur when a freshman or sophomore has a senior or two in their playing position, and the coach does not want to 'waste' a year of eligibility just for some random playing time. These situations are talked about either before the fall practice starts, or at the very latest, during the pre-season training.
2) Medical or Injury redshirt - This is where a player suffers a season ending injury. The injury must occur before the halfway point of the season, and before the player has played in approximately 30% of the matches. This usually equates out to about 6 -7 matches of play. The tough situation is when the player is a starter, plays in 10 or so matches before the halfway point, then has an injury and can't play the remainder of the season, they still can't redshirt.