It is the practical upper bound on circulating density, reported in psi per foot or as an equivalent mud weight. A leak-off test at the casing shoe is the usual measurement in the open hole just drilled. That test is related to the minimum horizontal stress, but the leak-off pressure is not the same number as the closure stress: it includes the pressure needed to start the fracture, not only to hold it open.
Planning a casing point is the act of keeping pore pressure and this gradient apart by enough mud-weight window to drill the next section. A window that has closed is a casing point whether or not the programme had one there.