Well continuity is for connected well as it is going to tied for global vss. As the standard cells are individual with no well ties in the cells. Once they are put together then well has to be connected. and this applies to N-Well and P-well and power continuity. This is when at the top level there is overall connectivity of the wells/power. Care has to be taken for the connection at cell level design to connect all the cells for well and power connectivity when things are assembled.