The diagnosis of hypertensive emergency is made based upon the findings of hypertension in the setting of end-organ damage (usually heart, brain, or kidneys). What you really care about is the rate of increase rather than the actual BP number (there is no defining threshold).
A Hypertensive emergency is any elevation in blood pressure in the presence of end-organ dysfunction. Aggressively treating severe asymptomatic hypertension (very high blood pressure without clear end organ damage) is not indicated, not supported by the literature and DANGEROUS. First, do no harm.