A flexible share-counting provision in a stock plan to determine how many shares have been used and how many remain. Stock awards with a higher accounting value are weighted more heavily against the plan's share reserve than lower value awards.

For example, while stock options may reduce the plan reserve by one share for each option granted, restricted stock or RSUs ("full-value awards") reduce the pool by a greater number (e.g. two shares for each RSU granted) because they have a higher accounting value under ASC Topic 718 (formerly FAS 123(R)). This also reflects the higher value/cost that institutional investors and proxy advisory firms (e.g. Risk Metrics) place on full value awards when evaluating a company's stock plan.

In case you were wondering, fungible is a legal term that essentially means "interchangeable."