Inside a chip we connect all Flip Flop through scan chain. This assures there is no stuck at 0 or stuck at 1 in any Flip Flop so that in functional mode it can work properly.
I am not sure what you are referring to by functional scan.
functional scan could be a scan stitched netlist where the data can be shifted in and out.
If you are referring to functional patterns, it means the patterns which are developed through functional tests and not atpg tool