This document discusses the Java Virtual Machine (VM) transport in Mule for intra-JVM communication between flows. The VM transport uses in-memory queues by default but can be configured for persistent queues, though VM file persistency does not work in clusters. It describes the VM transport having two modes - request-response which delivers messages directly between endpoints in the same thread, and one-way which uses queues for asynchronous delivery. Features like persistent queues using the file system are also covered.