What is the difference between a subscribing and a tracking event processor in Axon?

A subscribing event processor receives events synchronously, on the same thread that published them, the instant they're applied — the handler runs inline, before the originating command is considered fully finished. A tracking event processor runs asynchronously on its own thread and keeps a durable token recording exactly which event it last processed, letting it catch up at its own pace. Because the command only waits for the event to be stored, not for a tracking handler to finish, tracking decouples projection speed from command speed. Tracking processors can also resume from an exact position after a restart and can replay event history on command, neither of which a subscribing processor can do. Axon defaults new processors to tracking mode precisely because these benefits apply to most real projections.