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PRMan has the ability to resume interrupted renders. How this works depends on the mode that it's running in. Non-incremental renders to TIFF and OpenEXR images can always be recovered. Incremental renders to OpenEXR images can also be recovered, but only if the checkpointing option was used.
The following options are all available in the 21.8 release. The same is available in prior releases of 21, excluding the .part, compression, and deep data.
Checkpointing is specific to the incremental rendering mode when doing batch rendering. In incremental mode, the renderer makes repeated passes over the image, refining it a bit more with each pass. While the image will be quite noisy during the initial passes, it is usually sufficient to give an impression of how the final image will look.