Back/Lay Ratio is important rule that calculates difference between available Back and Lay price for single selection. When Back price is close to Lay price then the offer is valid and good e.g. Back price 2.20 and Lay price 2.22 is best possible offer on market for selection as there are no possible prices between 2.20 and 2.22. If the gap between Back and Lay price is big then offer is not valid e.g. Back price 2.0 and Lay price 9.0 is very bad offer as bot cannot be sure which of those two prices is valid one.
Back/Lay Ratio rule is added by default to each strategy and it should never be removed. Only in cases where your strategy is set to bet at high prices you might want to set this rule to be less strict e.g. to allow difference of 30% or 50% between Back and Lay price, but at smaller prices difference should never be more than 15%.
This rule should always be used in combination with Overround rule and Min/Max price rule.