Does the Dell OptiPlex 5000 SFF use desktop or laptop RAM?

The Dell OptiPlex 5000 SFF uses desktop UDIMMs, not laptop SO-DIMMs. It takes full-size DDR4-3200 UDIMMs with two slots and a 64 GB ceiling, non-ECC. If you have spare desktop RAM, it might work here. Dell reserved SO-DIMMs for the Micro line and gave the SFF proper desktop sticks.