Armour buy limits, page 6

How the 4-hour buy limit works

A buy limit caps how many of one item you can buy on the Grand Exchange in 4 hours. The clock starts at your first purchase of that item, not on a fixed schedule, and every unit you buy inside those 4 hours counts. When the window ends the full limit is available again. Selling is never limited, and selling does not give any of the limit back. Limits are per account.

The line under the buy price is the cost of one limit: the buy limit times the price, the gp a full limit ties up. Profit per limit is the buy limit times the profit per item after tax, an estimated ceiling for one 4-hour window. It is only shown when a full limit is no more than 10% of what trades in a typical 4 hours, the same share the flips board assumes you can capture; otherwise it reads “Low volume”. A profit needs both sides to have traded in the last 2 hours. The line under the buy limit is the limit as a share of the units traded in the last day: above 100% means a full limit is more than the whole market traded.