Lexical choice means nothing more than word choice. It's clearly an important aspect of creating a suitable style or register (i.e. when making language choices suited to a particular genre, context, audience and purpose). It might be easier to think of lexis as referring only to the form of a word - its shape and sound as opposed to its content or meaning. So a text containing many polysyllabic word choices, for example, would suggest it is suited for a more educated or older speaker; another example, rhymed word choices might suggest a text written in a style suitable for a younger child, and so on.