There are some cases where you can be invited for a voluntary interview where they haven't established enough for arrest if you refuse. Its a somewhat confusin

There are some cases where you can be invited for a voluntary interview where they haven't established enough for arrest if you refuse. Its a somewhat confusing term, but that's what they call it. Usually its made clear to you at the time which of those cases its going to be. "Look, you can come in for an interview at a time that suits both of us, or we will have to go and get you at a time that suits us. Either way, you will be interviewed."