Back Strap

Is it ok for the strap in the back of a formal vest to show?

It is the strap in back that you can pull to make it tighter.
Is that ok to show or is it suppose to be covered by a jacket.
And heard that if it is backless then you should leave the jacket on, but I don't know what backless means.

There are two types of vests. The "backless" style has a front that goes over your head and rests on the back of your neck. The "back" is a network of straps because this style is a false front--the vest is truly "one size fits all." The straps are just plain elastic and don't look like much, so you have to keep the jacket on. The "full vest" has a fancy front and a back in a coordinating-colored satin. The only adjustment is a single horizontal strap to tighten the waist. It can be worn without a jacket. This vest must come in sizes--s, m, l,xl, and involves inventory. Tux rental places prefer the first type.