"SOLIDWORKS PDM Professional 2018 supports SOLIDWORKS 2018, 2017, and 2016. SOLIDWORKS PDM Standard 2018 supports SOLIDWORKS 2018 only"

I wonder why this restriction exists in PDM standard compared to professional?

So another down side of PDM standard is that you have to go through the PDM upgrade with every solidworks upgrade. There is no chance to "get ahead" with the pdm to cut down the frequency you must go through the considerable complexity of the PDM server upgrade.