They are demanding the whole report come out with all supporting documentation used to reach his conclusions. The reason is they are going to thoroughly examine every bit of supplementation to pick the findings apart. Raise doubt in how Mueller could have come to a certain conclusions based on supporting evidence that shows the contrary. It is a great trial litigation technique used when trying to discredit a report, person, methodology, etc. Lawfag here and have done so myself in trial. Ripped apart an expert witness in trial before a jury and the opposing side then starting ripping into their witness as to why he was not a good expert witness and never should have been used, blah blah blah. You get the point.
LB sorry forgot to add.