A hydraulic fracture opens against the least principal stress. In a relaxed basin that least stress is this horizontal one, so the fracture is vertical and the closure pressure measured after a fracture treatment is an estimate of it. The fracture gradient used for drilling is related, but a leak-off test includes the extra pressure to start a fracture and is not this stress by itself.
It is reported in psi or as an equivalent mud weight, always against true vertical depth. A stress direction matters as much as the magnitude: the fracture propagates in the direction of the larger horizontal stress and opens against this smaller one. A wellbore-stability window and a fracture design that use different values for it will not describe the same rock.