Morphology is the study of form. Polymorphism is about objects that can have a plurality of forms. To illustrate polymorphism consider a double precision floating point value. It can be viewed as a 64-bit value or as a composition of a sign, a biased exponent and a significand. If the biased exponent is a maximum and the significand is zero, the value is infinite. On the other hand if the significand is non-zero, the significand morphs into a logical value where each bit signals a different not-a-number condition. When the biased exponent is between 1 and max-1, the significand morphs into the ratio of a numerator to an implied denominator. The point I am making is that the meaning (i.e. semantics) changes as a function of the content.
Now let us apply polymorphism to a stream of bytes. The content which changes the meaning of the bytes is called a policy indicator. Policy indicators perform two important functions: they separate elements within a byte stream and they specify the form into which the element will morph. (i.e. will it be UTF-8 text? a real-number? an angle? an unsigned integral value? a logical value? a Boolean value? a byte-array etc.)