Specifically, looking at the posts for naked_sweater, 99% of the time the sweater extends past the hips, making one unable to tell if the character has any panties on or not. As I read the bottomless and no_pants tags, bottomless is for when a character is wearing no clothing below the waist, while no_pants is for a person wearing no skirt or pant type clothing, but still has underwear or similar on.
What is the general rule on this type of situation? With post what you can see, it almost seems to me like neither would apply, since you can't verify if the character has panties on or not; And if there are panties, bottomless wouldn't apply, and if there are none no_pants wouldn't apply.
As is just using the example from above of naked_sweater, there are currently 14 pages that come up from the tag, but only 4 if you do "naked_sweater -bottomless", with most of the 10 pages that are lost with the - qualifier containing mostly images with those ambiguous situations (though not all of course).
Updated by S1eth