There are four kinds of interactions that we have with police.
I like to think of them on a spectrum.
We'll start with the lowest which would be a (1) voluntary encounter.
(Example) We're standing in line next to a police officer at a grocery store and you start talking that's a voluntary encounter.
Then we have a (2) detention. It's very common with the traffic stop if you're stopped in traffic you are the subject of a detention.
Then we have (3) custody. Custody is when you're brought in and your freedom is significantly restrained and you're being held to answer for a charge.
And then of course at the top of the spectrum is (4) arrest.
Now 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 and 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 this case lies.
Generally speaking, the prosecution wants to move everything to the far end and say it's a voluntary encounter. Because if it's a voluntary encounter then the police do need a 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 the burden is to show that they have to have evidence that you've committed these offenses.