For small assets: 3.5cm to 50cm - they will not have a Ztool file.
For regular assets: All assets will have a Ztool file. The only exception among regular assets which won't have a Ztool file are "packs" or "scatter" assets.
The source Ztool file will work with all versions of Zbrush. It includes several subdivision levels and has clean UVs, and utilizes the same texture maps as the other models. The Ztool can be used to generate other mesh variations not covered by the default downloads. For example, the High.obj/fbx file is capped at around 1 million polygons. The Ztool is generally much higher than this and approximates the original scan data. It could also be used as a basis for sculpting extra custom models if required.