Conditional page break
A conditional page-break will start a new page only if the break occurs outside the page-fill-area.
Roughly spoken you may say that it is an instruction for MusiCAD that says "if you're in need of a page-break, best do it at this position but if you have enough paper left just forget it."
If you insert conditional page-breaks at smart locations it will be more likely that your scores will print nice with different layout settings.
Use the button 
from the bar line menu or <shift-j> to insert a conditional page break.