insurance zone

What is the "gray zone" of insurance billing?
I work in human services to indigent or homeless. Another agency Mental Health provides free services "gray area", the billing insurance companies. My clients do not subscribe to the companies insurance, so how can bill their services? Apparently this is what the billing is the gray area, but I would like more details.
Never heard of him. Is this in reference to what kind of insurance?
DANGER ZONE LOOK!!!