This excerpt taken from the FFIV DEF 14A filed Jan 7, 2005.
Stock Units. Each stock unit agreement will contain provisions regarding (1) the number of shares subject to such Stock Award, (2) the purchase price of the shares, if any, and the means of payment for the shares, (3) the performance or other criteria, if any, that will determine the number of shares vested, (4) such terms and conditions on the grant, issuance, vesting and forfeiture of the shares, as applicable, as may be determined from time to time by the Board of Directors or other administrator of the 2005 Equity Incentive Plan, (5) restrictions on the transferability of the Stock Award, and (6) such further terms and conditions, in each case not inconsistent with the 2005 Equity Incentive Plan, as may be determined from time to time by the Board of Directors or other administrator of the 2005 Equity Incentive Plan. In the event that a participants relationship with the Company terminates, the Company may reacquire any or all of the shares of Common Stock held by the participant which have not vested or which are otherwise subject to forfeiture conditions. Stock units may be awarded in consideration for past services. Rights under a stock unit agreement may not be transferred other than by will or by the laws of descent and distribution.