How are publications outside Unit of Assessment assessed for REF? My understanding of the purpose of REF Units of Assessment (UoA) is that they determine what sort of subject specialist evaluates a REF return. Please correct me if wrong on this point.

One corollary of this is that Journal Impact Factors for publications are naturally assessed in context of their field, because the subject specialist understands how their field works.  For example, there are low-JIF fields such as History (mean IF=0.416 in table 3 of this 2004 paper) and high-JIF fields such as Molecular Biology (4.763), so a paper published in a journal with JIF = 3 might be seen as a great history paper but a lousy molecular biology one.

Obviously research outputs should be assessed on their own merits, but realistically, JIF of the publication plays a part in this.

Suppose a researcher does a lot of work outside their UoA - for example a computer scientist collaborating with researchers from several other disciplines to produce outputs in other fields than CS.

Are they able to submit these publications for their CS department's REF return? And how are the publications assessed if JIF norms are different for the fields where they have been collaborating?
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.