There are four kinds of interactions that we have will police. I like to think of them on a spectrum. We'll start with the lowest which would be a
(Example) We're standing in line next to a police officer at a grocery store, and you start talking - that's a voluntary encounter.
It's very common with a traffic stop. If you're stopped in traffic, you are the subject of a detention.
Custody is when you're brought in, your freedom is significantly restrained, and you're being held to answer for a charge.
As we move from a voluntary encounter up to arrest, the burden on the government increases. The burden of proof on the government increases.
So, in order for you to be detained, they have to have reasonable suspicion. In order for you to be placed in custody, they need to have even more evidence that you're involved in something.
Ultimately for you to be arrested they have to have established probable cause.
As we go up along this spectrum, your rights increase, and the level of evidence that they need increases.
Very often in a motion to suppress, we're arguing about where on this spectrum a case lies.
Generally speaking, the prosecution wants to move everything to the far end and say it's a voluntary encounter. If it's a voluntary encounter, then the police only need reasonable suspicion, but they don't need any evidence that you did anything.
Likewise, as the defense, we're trying to move things as close to arrest as we can. When that happens, you have increasing constitutional protections and an increased evidentiary burden on the government.