cross noseband

Question
hello ma'am, when should i get to know that i should use a cross noseband on a horse?

Answer
I think you are writing about a noseband that we call a figure eight noseband. Here (USA) it can only be used for jumpers or when jumping in an event. They cannot be used for regular showing or dressage.

They are used for control. I personally do not like them. They sometimes slip, and some people have a tendency to make them very tight. I do not have any students using them. They have trained their horses so that they do not need that additional control.

However, that said, if I had a student going cross country and her horse was not yet dependable and was tacked with a figure eight noiseband before she bought the horse, I would advise her to keep on using it for a while. Safety comes first.