How are publications outside Unit of Assessment assessed for REF?
How to assess interdisciplinary research is a known issue with the REF process. There is currently a REF Interdisciplinary Research Advisory Panel working on the details of how to properly assess outputs (papers, chapters, books etc) that cross the boundaries between different units of assessment. The panel is due to report what the detailed arrangements should be "in Summer 2018", so hopefully more details will be available shortly. 

What we know at this stage is that:


  Each sub-panel will have at least one appointed member to oversee and participate in the assessment of interdisciplinary research submitted in that UOA, with a specific role to ensure its equitable assessment. This role will include liaison with corresponding members on other sub-panels


A little more detail about the role of these 'interdisciplinary research advisors' can be found in a blog post on the HEFCE website from November.