Margin Call is an allowed margin level (40% and lower). At this point, the company is entitled but not liable to close all the open positions of a client due to the lack of free margin.
Stop Out is a minimal allowed level of margin (20% and lower) at which the trading program will start to close client’s open positions one by one (the first position closed is the one with the biggest floating loss) to prevent further losses that lead to negative balance (below 0 USD).