In REF 2014 SP11 allow a 100 word statement for each output. These were supposed to just cover significance, although some institutions interpreted this liberally and were not penalised. Some institutions sued the 100 words to the full with clear careful, possibly professional, editing for each. Others only rarely included a statement where there was something really outstanding for an output.
Look out for precise guidance as it become available, but I will assume REF2021 will pretty much follow 2014 pattern..
Remember that the panellist is about to read the full paper,do nottry to squeeze a mini-abstract into the 100 words. At best this is redundant, at worst obfuscates original items that are in the 100 words, but not in the text of the output.
Use the 100 words for two main things:
First use it to focus a non-expert reader on the contribution and significance of the work. Someone close to your area will recognise why the paper is important, but those more distant may not recognise this. If the abstract is well written tis may already be evident, in which case this will allow more room for the second aspect.
The second and most crucial thing is to provide evidence of the value of the work that is not contained within the paper. Examples of this include best paper awards, keynotes, external impact on other people’s work, follow on work, grants, patents, etc.