Processing is used for export and disk-caching certain Workflow Manager nodes. Please note that it is not a requirement that clips be disk-cached at all as the image data for an un-cached clip will be generated "on-the-fly" as required by PFClean. It's only if the time to achieve this is prohibitive, for example preventing real-time playback during review, is it necessary to cache the clip to disk.
Depending upon the configuration, processing can range from simply exporting/caching clips in the background whilst PFClean is idle to automatically distributing clips across a network of machines to make maximum use of available resources.
The common situation is for PFClean to perform all processing on the same machine, i.e. a non-distributed setup. The Distributed Processing section in the Project Manager is used to setup local processing and network processing, if available.