Storage
The Compositor adds a new storage location to your content_root called composites. In this directory all composites are stored, if you render to a composite name that already exists it will be overwritten. The composite data is stored as a single multi-layer .mig image file and a side-car .json file for metadata.
All image data is stored as uncompressed 32-bit floating point data which allows for fast loading and proper linear combination of the results. This also means you can change the tone-mapping paramters on the fly even at the compositing step.
You can move composites from one server installation to another simply by copying the .mig and .json file associated with the composite and then using the appropriate composite_name parameter. You should not rename these files.