While in sensory cognition the content as present in the mind and in the thing is identical (and thus numerically one content only), in rational cognition, where the concept is (gradually) formed, the content of the concept is not necessarily identical with some intended content in the extramental thing, but is in most cases only an approximation (so, for example, the concept of ' energy ' ).
In sensory cognition of an extramental material thing some content in the mind becomes formally identical to some content in the extramental thing : decoding encoded information (this guarantees objective knowledge). But, precisely what content (out of the many contents of the observed thing) is being identified will not be explicitly clear to the observer. Eventually many such contents will be so identified, that is, many contents in the extramental thing will become internalized in the mind. And only after repeated observations, experiences, and comparisons, these contents will be, as a result of deliberation, amalgamated or separated from each other, resulting in groups or clusters of them, and finally resulting in a universal concept. So while the preceding elements of a concept are infallible, the concept itself (as to what it precisely signifies) is not.
My concept of ' a substance ' for instance (as was worked out throughout this website [especially in First Part] ), could be wrong (Indeed, it is a 'difficult concept'). Also my concept of ' a crystal ', although not so difficult, could be incomplete or in some way biased.