Module D groups "benefits and loads beyond the system": essentially the environmental gains from recycling, reuse or energy recovery at end of life. It is calculated separately because the current rule does not add it to modules A, B and C in the regulatory calculation. This is an open methodological debate: manufacturers of recyclable materials consider it underweighted, while LCA scientists warn of double counting. For an engineering consultancy starting out, the simple rule is to display D for information and exclude it from the Ic Construction score.
Read the source article: EPD, FDES, PEP, LCA: what is the difference in RE2020?