Comments in Crucible can be used to flag a defect in the code under review.
To do this, simply check Defect when adding a comment and select a category from the drop-down list.
You may want to mark comments as defects in order to associate defect classifications, or simply to highlight to the author or moderator that the issue you raised in your comment requires attention. You can use the with defects filter to find files that have been flagged with defects.
Crucible intentionally does not mandate how defects are to be used. The Crucible administrator can customise the defect classifications.
You can only use the defect classifications on comments that are not a reply to an existing comment.