diffrence between seepage and piping?

Seepage is the movmnt of water under the ground , & it happens in any structure or naturally W/O str.

Piping AKA undermining , happens under Hydraulic structures ( i mean Downstream) mostly , it happens when the slope of the gradient of the water pressure is more than the critical exit gradient "i exit"

and whats called boiling for the sand case happens , its about boilig like eruption of water happening & that exit point , taking the fine materilas away (Washout) & corrosing the soil through time leading to failure the factor of safety agains Piping is F.S = i exit / i critical

P.S : the water pressure under the ground path is to be calculated using the Flow Net method