Air carrier pilot reported confusion attributed to not understanding ATC clearance; and ATC altitude...

Narrative:

On arrival to bwi we were cleared to descend via the anthm 3 RNAV arrival; landing east. As the PF I selected 4;000 (the bottom altitude at starz) and managed descent. Prior to eagll we were given a revised clearance to descend to 11;000 feet for traffic. Eagll has a crossing restriction of below 13;000 and above 12;000. As the PF I selected 11;000 and selected open descent. Several minutes later; but still prior to eagll; ATC gave us yet another revised vertical clearance; to descend to 11;000; then descend via the anthm 3 RNAV arrival; landing east. So as the PF I again selected 4;000 (the bottom altitude at starz) and managed descent. The plane slowed its descent to 1;000 FPM as we were now below the planned descent path. The pm who was distracted with other tasks looked up and queried whether we should still be descending at a faster rate to 11;000; and suggested we do so. The clearance was a bit confusing - we were not told 'best rate' (or anything like that) to 11;000; but it seemed plausible since it was 'for traffic' so I went back out of managed descent to increase the rate of descent; planning to again switch to managed at 11;000 for the further crossing restrictions after eagll. At 11;000 I selected managed descent again. Between 11;000 and 10;000 (as the plane was slowing its descent rate in the transition back managed) ATC queried us if we were descending on the anthm 3. At that point both the pm and I noticed that there was a crossing restriction of 11;000 at roktt; two fixes after eagll. Unsure if we should climb back up to 11;000; ATC gave us a new clearance to maintain 10;000; then descend via the anthm 3. Personally; next time there is a confusion about a clearance I will stop all other tasks so I am not distracted. If necessary I will query ATC. Externally; ATC should not give confusing altitude clearances in a terminal environment with a high volume of traffic.

Original NASA ASRS Text

Title: Air carrier pilot reported confusion attributed to not understanding ATC clearance; and ATC altitude changes from the procedure while on the ANTHM 3 RNAV arrival to BWI airport.

Narrative: On arrival to BWI we were cleared to descend via the ANTHM 3 RNAV arrival; landing east. As the PF I selected 4;000 (the bottom altitude at STARZ) and managed descent. Prior to EAGLL we were given a revised clearance to descend to 11;000 feet for traffic. EAGLL has a crossing restriction of below 13;000 and above 12;000. As the PF I selected 11;000 and selected open descent. Several minutes later; but still prior to EAGLL; ATC gave us yet another revised vertical clearance; to descend to 11;000; then descend VIA the ANTHM 3 RNAV arrival; landing east. So as the PF I again selected 4;000 (the bottom altitude at STARZ) and managed descent. The plane slowed its descent to 1;000 FPM as we were now below the planned descent path. The PM who was distracted with other tasks looked up and queried whether we should still be descending at a faster rate to 11;000; and suggested we do so. The clearance was a bit confusing - we were not told 'best rate' (or anything like that) to 11;000; but it seemed plausible since it was 'for traffic' so I went back out of managed descent to increase the rate of descent; planning to again switch to managed at 11;000 for the further crossing restrictions after EAGLL. At 11;000 I selected managed descent again. Between 11;000 and 10;000 (as the plane was slowing its descent rate in the transition back managed) ATC queried us if we were descending on the ANTHM 3. At that point both the PM and I noticed that there was a crossing restriction of 11;000 at ROKTT; two fixes after EAGLL. Unsure if we should climb back up to 11;000; ATC gave us a new clearance to maintain 10;000; then descend via the ANTHM 3. Personally; next time there is a confusion about a clearance I will stop all other tasks so I am not distracted. If necessary I will query ATC. Externally; ATC should not give confusing altitude clearances in a terminal environment with a high volume of traffic.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1631492
Time
Date	201903
Local Time Of Day	1801-2400
Place
Locale Reference	BWI.Airport
State Reference	MD
Aircraft 1
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Person 1
Function	Pilot Flying
Qualification	Flight Crew Instrument Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	ATC Issue All Types Deviation - Altitude Crossing Restriction Not Met Deviation - Altitude Overshoot Deviation - Procedural Clearance

Air carrier pilot reported confusion attributed to not understanding ATC clearance; and ATC altitude changes from the procedure while on the anthm 3 RNAV arrival to bwi airport.