Large Transport
Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Confusion; Communication Breakdown
ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Provided Assistance; Flight Crew Returned To Clearance
Procedure; Human Factors
I flew flight from ZZZ to BOS as Pilot Monitoring. We were expecting to fly the ROBUC1 to runway 27. While on BOS Center we were told to descend to FL230. Subsequently; we switched to BOS center (freq 134.0) where we were told to 'Go direct KRANN and to fly the RWY 27 transition'.  Familiar with the arrival; we had not heard this phraseology. We misunderstood and thought we were cleared to descend and fly the transition and started to descend to be over KRANN between 11;000 FT and 13;000 FT and above 8;000 FT at ANSLY etc. as hard arrival published altitudes (not expects).Center observed us descending and had us level off at 21;000 FT. I inquired as to what 'cleared to fly the transition to RWY 27' means; did it mean we could fly the track and not the altitudes or the speeds? The response was that he didn't know and the controllers were waiting for training before they could give a 'Descend via' clearance; that all he could say was 'Cleared to fly the transition' and that the pilots were supposed to know what that meant. Once I realized ATC had not actually said 'Descend via' I realized we should have questioned the clearance before descending. As soon as we switched to approach we were given 'descend via the 27 transition'. Landed without incident. We expected to be cleared to 'Descend via' the approach then when we got different wording we 'Assumed' that we could fly the SID as published but obviously that wasn't what was expected. Listening more closely and question when any other words from the controller are heard. This is new to all of us; controllers and pilots alike.
I was pilot flying the ROBUC1 arrival into BOS. We were in the vicinity of JFK level at FL 230 and ATC cleared us direct Krann and fly the runway 27 transition. I misinterpreted this to mean that we were cleared to descend to meet the altitude restrictions on the arrival and began the descent. ATC noticed the descent and reassigned us FL210 and pointed out that we were not cleared to begin the descent. We were then handed off to BOS approach and then cleared to descend via the ROBUC1 and fly the runway 27 transition. We resumed the descent on the arrival and landed without further incident.Avoiding future events can be accomplished by paying closer attention to the exact ATC phraseology of these new arrival procedures.
Pilots flying into BOS were told to fly the Runway 27 transition and they misunderstood and thought they also had descent clearance on the approach.