An Onload 'stack' is an instance of a TCP/IP stack. The stack includes transmit and receive buffers, open connections and the associated port numbers and stack options. Each stack has associated with it one or more Virtual NICs (typically one per physical port that stack is using).
In normal usage, each accelerated process will have its own Onload stack shared by all connections created by the process. It is also possible for multiple processes to share a single Onload stack instance (refer to Stack Sharing), and for a single application to have more than one Onload stack. Refer to Onload Extensions API.