CSFF - cold soaked fuel frost
on the upper surface of the wing there are those black lines indicating where CSFF is permissible.
from the book:
Takeoff with light coatings of frost on upper wing surfaces due to cold fuel (cold-soaked fuel frost) is allowable, provided the following conditions are
met:
• the frost on the upper surface is less than 1/16 inch (1.5 mm) in thickness
• the extent of the frost is similar on both wings
• the frost is on or between the black lines defining the allowable cold-soaked fuel frost area (see figure) with no ice or frost on the leading edges or control surfaces
• the ambient air temperature is above freezing (0°C, 32°F)
• there is no precipitation or visible moisture (rain, snow, drizzle or fog withless than 1 mile visibility, etc.)
If all the above criteria are not met, all ice or frost on the wings must be removed using appropriate deicing/anti-icing procedures.
Note: If the frost on the lower surface is less than 1/16 inch (1.5 mm) in thickness, the frost on the upper surface will be less than 1/16 inch(1.5 mm) in thickness.