When is probation not the same as a suspended sentence

The answer is when you get probation in federal court, according to the Fourth Circuit's holding in U.S. v. Pollard, in which the Court in an opinion by Chief Judge Wilkins joined by Judge Traxler with District Judge Titus dissenting concluded that there was no violation of the constitutional right to counsel where the defendant was given probation for a speeding ticket (among other charges) at Quantico Marine Corps base, then faced incarceration when he got another ticket a few days later.