Hi
Before answering this question, we need to understand what is meant by distal and proximal. The terms distal and proximal in biology are used to refer the sub parts of the structures which contain a proper beginning and an end, such as our blood vessels, or upper and lower limbs etc. Actually in this types of structures the parts of the structures can be described with reference to the start or origin of the structure, for example: Proximal part of the arm near elbow bone etc.
Now coming towards the question that why the structures like Chin can not be described with the help of these two terms, this is obvious that nose, chin, nails etc donot contain significantly describable distal and proximal ends either due to their out of the context shape or less size, therefore, the terms cannot be used for them.
Hope it helps!
