EXR multi-part output.

Improved read-speed in comp software. Vray has it:
"EXR multipart – When enabled, V-Ray saves the multichannel OpenEXR files in a format that is read much faster. Note that this option is only compatible with OpenEXR 2 and above."

Use case:
We are animation studio and one project usually consists of 20-30 animated shots. We need multiple output elements for control and flexibility. We currently render 15+ render elements and output is singe multi-layer EXR file per frame (it contains all the elements). Most of the shots dont need all of the shots so our workflow and compositing software would benefit from multi-part format by reading only the elements we need for individual shot and not the whole exr file.

More info about multi-layer vs multi-part:
https://medium.com/@stragalet/exr-1-0-vs-2-0-c438ce1efaf4