Symbolic work is primarily intellectual or artistic. It 'produces ideas, symbols, codes, texts, linguistic figures, images, and other such products' (Hardt and Negri 2005, 108). It includes computer programming, graphic design, various sorts of media work, work in advertising, and public relations, etc.. Work of this kind, it is true, does not directly create a material product. In this respect it resembles commercial, administrative and other kinds of service work. However, it is wrong to think that a new category of immaterial labor is needed to comprehend it. The error here is to imagine that 'symbolic' work of this sort has no material result and that only work which directly creates a tangible product, like industry or craft, is material activity. [...] All labour operates by intentionally transforming matter in some way, as Marx maintains. Symbolic labour is no exception: it involves making marks on paper, making sounds, creating electronic impulses in a computer system, or whatever. Only in this way is such activity objectified and realized as labour.
Sean Sayers with a very good critique of Hardt and Negri's concept of "Symbolic labour" in Marx & Alienation, Essays on Heglian Themes, Ch. 2, The Concept of Labour