Here the options are 'front projection' and 'rear projection' which are self explanatory utilising projectors positioned in front and behind the screen respectively.
As usual, both systems have specific strengths and weaknesses.
Front projection on the one hand requires more set up to align the interactive whiteboard with the image, and then requires the presenter to stand in front of the projector where they will cast a shadow over the whiteboard screen.
Most users compensate for this drawback by standing just to the left or right of the interactive whiteboard and 'reaching' in.
However this limits potential functions, and also means that those presenting are required to look into the bright light in order to address the audience which can be disorienting and unpleasant.
Those that use rear projection meanwhile are generally built into the machine itself and require no alignment as such.
However this also makes the screens very large and thick and means that they cannot be wall mounted (unless they are specifically built into the wall, which requires a more complex installation which is more expensive) and are generally more expensive.