The traditional tour bus open topper was usually either a restored heritage bus, or a converted standard bus. Sometimes the bus is converted if its top has been damaged by hitting a low obstacle e.g. a bridge. Tour operators sometimes export a double-decker bus from the United Kingdom, and convert it to an open top bus. This is to give the impression of an archetypal British bus, such as the Routemaster London bus, although often the bus actually purchased is not a Routemaster.
In recent years with the growth in tourism, open top bus designs have become available to be purchased new, often with long multiple axle and low floor Easy Access features. In the past, and in smaller companies, open top buses were converted from regular service buses, sometimes after a de-roofing bridge strike type accident.
The open deck in an open top bus may have the roof partially or fully removed, with a guard rail. An intact roof section may be left at the front or rear, and the full/half window height may remain in the open sections. Some open top buses may feature a re-fittable roof section, for fitting in inclement weather. Some may also feature upper deck seats specially outfitted to withstand the elements.
Source: Wikipedia Open Top Bus Article