Depends what was taken out. If they were timber frames, the fitters split the frames in half and take a piece out of the middle. This gives them room to wiggle the old frames out. Now the new upvc frames have got to be got into an opening that was probably "finished" around the first frames ever fitted. the outside reveal will not take the frame and the cavity, if there is one, is not wide enough to present the window into the opening and move sideways into position. One edge of the reveal is chiselled away to allow presentation of the new frame. When the new frame is in place it is sealed with insulation(usually foam) and the side reveales are trimmed up with matching upvc to leave a neat finish.