Sentence:  In these sheds, individual products rest on short racks, so they can be reached more easily by employees who pick and box orders. In order to fit more racks, companies put in several <WSD>mezzanine</WSD> levels. As a result, the sheds rise taller: 21 metres, compared to B2B's 15 metres. 
Meaning of the word marked with <WSD> tags:  Additional flooring laid over a floor to bring it up to some height or level.