Possible Clues:

• Meas. of blocking
• Rating for cover applications: Abbr
• Sunscreen meas.
• Stat on a Banana Boat bottle
• Letters on some ocean lotions
• Beach tube letters
• Letters on ya sunscreen
• 50+, e.g., on a L'Oréal tube: Abbr
• Hawaiian Tropic lotion letters
• UV-protection measure