It is not subject to a substantial risk of forfeiture.
Property is considered to be transferable when it can be sold, assigned or pledged to another person without restrictions; that is, the person receiving it does not assume the risk of forfeiture.
A substantial risk of forfeiture exists if rights to the property depend on performing certain services (working a certain period of time, or meeting a goal or objective), or not performing certain acts (such as leaving the company), and there is a high risk of losing rights to the property if the condition is not met.