The concept of 'discourse' is the key to studying how semiotic resources are used to construct representations of what is going on in the world.
The concept of 'genre' is the key to studying how semiotic resources are used to enact communicative interactions - interactions that involve representation - whether face to face, as for instance in conversations, or at a remove of time and/ or place, as for instance through the means of books and other media.
The concept of 'style' is the key to studying how people use semiotic resources to 'perform' genres, and to express their identities and values in doing so.
The concept of 'modality' is the key to studying how people use semiotic resources to create the truth or reality values of their representations, to communicate, for instance, whether they are to be taken as fact or fiction, proven truth or conjecture, etc.
Although I discuss these four dimensions one by one, they never occur in isolation and are always all part of every communicative event and every semiotic artefact. Only looking at them together will give a complete, multidimensional picture.