R
It's exactly what it says on the tin, a statement in regard to the reasons for the Decision.
They can have relevance at the various levels of the Decision process, so at a Tribunal they are taken very seriously and a failure to produce one is Good Cause to have the panel's Decision Set Aside.
Unfortunately at the DWP end of the process, they have little value and a request for one might at best result in a copy of the assessor's report., you can request this yourself anyway, or more likely, just a referral back to the Decision Letter that is sent out to claimant's.
Have you requested a copy of the assessment report? It's a PA4 if you had an assessment you participated in, or a PA3 if it was all done on the papers.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems