IMF IAB.mxf files are designed to be used in IMF authoring applications to create integrated mezzanine deliverables that include Video (including Dolby Vision) and Dolby Atmos audio.

At present (May 2019) there are no encoding tools for encoding to IMF IAB.mxf files to Dolby Digital Plus with Atmos Content but these are in the pipeline for future releases on Dolby encoding products.   

For the near-term either .atmos or ADM BWF .wav will continue to be the deliverable based upon your customer's delivery specification.