The formation just below the shoe is where a leak-off test is usually pumped, and that test sets the fracture limit for the next section of hole. The shoe depth in the drilling report is a measured depth. The fracture gradient uses the shoe's true vertical depth.
'Pressure at the shoe' during well control is the pressure that must stay below the leak-off limit while a kick is circulated out. A shoe set too shallow in a pressure ramp leaves the next section with no safe mud weight.