There are several differences:
First of all, the plug-ins available with the BIMkit Free license are BK Tiles (for tiling) and BK Panels (for raised floors and suspended ceilings), while those available with the BIMkit DOC license are BK Gialli e Rossi (for comparative layouts) and BK Docfa (for exporting DXF files compatible with DOCFA).
Secondly, BIMkit Free plug-ins are available free of charge, whereas BIMkit DOC plug-ins require an annual subscription.