The "internal file format" for images has purposely been left undefined to allow us to choose the best available format to support our needs. It has always been a requirement for us to be able to export images as FITS files, although it is not absolutely required that all available information be placed in those FITS files (so that they could be "round-tripped").
It is becoming urgent for us to define what the image interface is between the DAX and SUIT teams. Since SUIT uses Java, the Butler and afw are not available to help intermediate. SUIT needs a serialized file format that is efficient to access from Java.
Mario and I propose that we baseline the choice of FITS as the image interface. This is what is currently supported on the SUIT side and already required to be output by the DAX side, so it seems like the lowest-cost choice.
The Science Pipelines do not need this level of baselining because they have the Butler to isolate them from the serialized file format. The Butler should still maintain the ability to support multiple serializations.
If future developments in file formats provide additional functionality that SUIT could use, this decision could be revisited.