Final answer:
A police officer needs no justification to engage in a consensual encounter, as these interactions are voluntary and the person is free to leave. Reasonable suspicion is needed for a Terry stop, while probable cause is required for arrests without a warrant and constitutional searches.
Step-by-step explanation:
The amount of justification needed for a police officer to engage in a consensual encounter is none. This is because a consensual encounter involves an officer interacting with a person without any force or compulsion, and the person is free to leave at any time. Therefore, no suspicion or justification is required.
During a consensual encounter, if a police officer develops a reasonable suspicion that a person has committed or is about to commit a crime, this can escalate to a Terry stop (or an investigative detention), as established by Terry v. Ohio. The officer can then stop and briefly detain the person. A reasonable suspicion is a lower threshold than probable cause and certainly lower than 'beyond a reasonable doubt' which is the standard used at trial.
Probable cause is needed for a police officer to make an arrest without a warrant for most felonies or for a search to be constitutional. This is also less than 'beyond a reasonable doubt' but more than 'minimal suspicion' or 'reasonable suspicion'.