Although deber usually translates as 'must' to express obligation to do something (read more), when talking about something that is a particular way in the present, comparing with what generally must be done, it tends to translate as 'should'. [See also debería vs. tendría que]
-Las bicis no deben circular por la acera — People mustn't cycle on the pavement / Bikes aren't allowed on the pavement [In general]
→Los ciclistas no circulan como deben — Cyclists don't use the road like they should (must??) ['Should' is often 'counterfactual', i.e. it tells us the things aren't the way they are supposed to be or the way we would like them to be]
→La gente circula por la acera aunque no debe — People cycle on the pavement even though they shouldn't (mustn't??)
-Las piezas deben ser así — The pieces must/need to be like this [In general; 'deber' often translates as 'need to' - see tener que vs. necesitar]
→Las piezas son así, como deben ser — The pieces are like this, as they should be
→Estas piezas son pequeñas; no deben ser así — These pieces are small. They shouldn't be like that
