As nouns the difference between manual and specification is that manual is a handbook while specification is an explicit set of requirements to be satisfied by a material, product, or service.
As an adjective manual is performed with the hands (of an activity).
As nouns the difference between specification and feature is that specification is specification while feature is (label) one’s structure or make-up; form, shape, bodily proportions. As a verb feature is to ascribe the greatest importance to something within a certain context.
As nouns the difference between characteristic and specification is that characteristic is a distinguishable feature of a person or thing while specification is an explicit set of requirements to be satisfied by a material, product, or service.
As an adjective characteristic is being a distinguishing feature of a person or thing.