What is the difference between xorg-x11-fonts-75dpi and xorg-x11-fonts-100dpi?

Under what circumstances would one want to use the 75dpi version versus the 100dpi version? If you have both installed, does the xorg server just pick the one that it considers best based on some criteria? Should you have both installed? Is anything gained by having both installed?

As you can see I don’t know much about bitmap fonts, but I’ve just started playing with the XForms library and discovered that CentOS 7 doesn’t install bitmap fonts any more.