"details" or "comment" means that the coin has a problem that does not affect the absolute grade.
For example, a coin with minor damage that it if didn't exist would result in a straight grade.
With the minor damage, the coin retains the grade, but the damage HAS to be commented on in all fairness.
If the problem is major enough, it is possible that the coin may not be graded at all. Such a decision not to grade can be somewhat subjective on the part of the TPGrader.