Slush credits are sessions that sit in a fund on top of your regular subscription monthly session allowance. Slush credits roll over from month to month and are designed to tide you over on the occasional month that you go over your monthly allowance but you are not quite ready to upgrade your plan. For example if you are on the 25 session plan and you have a busier than usual month and end up needing 27 sessions, the 2 extra sessions would automatically be drawn from your slush fund. Slush is only available with a current subscription and is not offered as a pay as you go option.