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Stop on opening bet

A stop on opening bet applies to the initial 'entry' bet of a trade as opposed to a regular stop loss that applies to the offset 'exit' bet.  As with other types of global settings, if you want to the switch to another market, you should add the current market to Guardian (which will manage the global settings in the background).
Using 'Stop on opening bet' adds a stop to the initial bet in an attempt to ensure the bet is matched even if the market moves away from your requested entry price. This can be useful if you wish to ask for a price that is better than is currently on offer by the market, but also want to make sure the bet is filled.  e.g. If the current market is available to back at 1.68, place a Back bet at 1.7 with an opening stop 5 ticks lower.  The bet will initially be unmatched in the market and the price may drift to fill your requested price of 1.7, but if it goes the other way and touches 1.65 then your opening stop will trigger and your Back bet will be repriced to 1.65 to try to fill at the current price.
'Stop on opening bet' could be used to add stops to dutching, bookmaking or make market positions. In the case of dutching, you may like to back at the lay price to gain that extra tick out of the market, but need to book to complete regardless (otherwise you haven't fully dutched the market and have an incomplete book).  For example, if you set a five tick opening stop using global settings on the dutching screen and your unmatched bet eventually fills at the lay price then you are in a good position. If it doesn't work out and the price comes in five ticks, the opening stop will open the position on the runner for you. Without the stop, the price could collapse in and your only way to ensure the fill would be to wait until the market turns in play and then Take SP.
If you're just placing a single bet you may wish to use the opening stop.  For instance, during an in-play soccer match, you offer to back the Home team two ticks above the current price as you want to place a back bet, but want more value for money. You think the price will spike in the next few minutes due to generally volatility in the game.  If it fills you get want you wanted, but if the price contracts by your specified amount then the stop triggers and your bet is still placed.  You didn't get the price you initially wanted, but at least you've still got your bet filled on the Home team automatically.