In short, if your computer has too much RAM installed, BeOS will crash.
R5.0.3 has a maximum of around 1024mb. R5.0.3+BONE and R5.1d0 Dano/Exp have a maxium of around 768mb. These maximums are further reduced by the amount of your video cards memory.
In order to work around this, a RAM Limiter patch has been created. It will trick BeOS into seeing only a portion of your total RAM, hence "Viewable RAM".
Basic formula for estimating your systems viewable RAM:
5.0.3: 1152mb - video card memory 5+BONE: 896mb - video card memory 5.1d0: 896mb - vidie card memory
If you wish to permanently install this patch, click here.