What does ENOB of 13.3 bit means in the specification sheet?

ENOB = (SINAD-1.76)/6.02
Overall Accuracy = ENOB (Effective Number of Bits)
So, in general, ENOB is a measurement that includes the consideration of noises which reduces the overall accuracy of measurement. Eg. A 14 bit device would have an ENOB of 12 bit because the other 2 bits would have been used up with noise measurements.