Physical requirements mean different things to different people. In the context of building your data center, I could go into minute details regarding actual construction of the facility, for example. In such case, physical requirements would exist with regard to the following:
Physical construction materials and related factors (fire codes, weight-bearing members for roof-mounted AC/Environmental Units, load-rating factors for the raised floor construction, cable risers or trays, and so on).
Location (avoid the first floor or top floor of buildings, and avoid flood zones, areas where access may be limited in terms of single stairs, elevators, roads, and so on).
Environmental systems (cooling, heating, humidity ...