Sdawg34,
Medical redshirt example:
My son is a D3 pitcher who is an academic junior but expects to take 5 years to graduate. He played his freshman and sophomore years and pitched this past fall, but is rehabbing an injury this spring. He will not play this spring at all and expects to get the medical redshirt waiver, so that he will have 2 years of baseball eligibility remaining.
However, another pitcher on my son's team got the medical redshirt waiver last spring even though he had already pitched in some spring games before the injury. I don't know the specific guidelines, but there is a certain number of games or innings that a player can participate in, up to a certain point in the season, and still be eligible to apply for the waiver for an injury. IF the waiver is granted by the NCAA, that player would not lose a year of eligibility for the few games or innings they played in.
I hope I explained that right!