The term „illustrative“ is typically used to degrade an artwork that wished to be „true“ art.
Some artist have tried to argue against this judgement and bridge the gap between art and illustration.
In the US those two realms are more closely linked and often taught at the same institutions.
Hence those artist have a different perspective on this matter.
However the international art market generally remains unsympathetic towards illustration, with some exceptions.
„True“ art refers to productions that are accepted in the discourse of the art word and the art market, they are not illustration. „True“ art means „pure“ manifestations of more than just the artist’s ideas, it’s their genius, the unconscious, the intuition, which is of a higher status and value.
The artwork is indeed a manifestation of the artist’s ideas, further than that, it is a manifestation of the artist’s being. But this is true for all creation, even for all action in general. Fundamentally manifestation is the joinging of Heaven and Earth in a symbolic understanding. On a side-note, manifestation of the material and non-material might be called emergence.
Therefor the artwork is not only a manifestation of the artist’s ideas, but equally and most fundamentally a manifestation of his bodily actions.
So an illustration is also an manifestation.
And since both are manifest, the criteria of manifestation is obsolete.
However I think that every work of visual art (maybe even non-visual art) is both, and manifestation and an illustration.
To illustrate, etymologically, means to illuminate, to shine a light on something.
This also applies to „true“ art, no one would object to that.
Why has illustration a negative meaning compared to art?
„Illustrative“ gets its negative connotation from its function in the creative field.
Illustration (as a genre in the artistic crafts) is the making of an image that refers to a preexisting text.
So when an artwork is called illustrative, it simply means it is describing a text, it is literal.