DotSimplifyRelations is an option for DotSimplify. Its setting may be a list of substitution rules of the form DotSimplifyRelations -> {a.b -> c, b^2 -> 0, ...}.
In the rules, Condition should not be used and patterns should be avoided on the right-hand sides.
Notice that the performance of DotSimplify scales very badly with the complexity of DotSimplifyRelations and the number of terms of the expression.