Binocular vision and heterotropia / strabismus / squint

Dingle_berry

One for the professionals:

"Normal binocular function is defined as any individual with a degree of binocular lock. This would include individuals with normal binocular single vision and those individuals with well-adapted heterotropias, who are not at risk of diplopia and have adopted a suppression scotoma when both eyes are open."

What is meant by 'a degree of binocular lock' in the above statement? and how can this be demonstrated to an optician / orthoptist / ophthalmologist by a person with a heterotropism?