The Highest Nudity Level Rating basically just lets the users know what is the most nudity they can expect to see on your Profile and content.

To determine the highest nudity level rating for a Profile, the website looks at the Profile Photo as well as the Photo Albums linked to a Profile. It then takes the most severe of the nudity ratings (No Nudity, Partial Nudity or Nudity) and sets that as the highest nudity level.

For example, if your profile photo and all your photo albums contain No Nudity, your Profile is assigned a Nudity Level of No Nudity.
If your profile photo contains No Nudity, but one of your photo albums contains Partial Nudity, then your Profile is assigned a Nudity Level of Partial Nudity. And so on.

This is only applicable to Profiles with a Profile Photo and / or Photo Albums.