Date: 201812
Local time of day: 0601-1200
Location: LAX.Airport, CA
Flight conditions: VMC
Aircraft: B757-200
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Climb
Airspace: Class E SCT
ATC / advisory: TRACON SCT
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Human factors: Confusion; Communication Breakdown; Situational Awareness
Second reporter function: First Officer; Pilot Not Flying
Anomaly: ATC Issue All Types; Deviation - Altitude Undershoot; Deviation / Discrepancy - Procedural Clearance
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: First clearance we received via CPDLC included a 'Climb Via SID' clearance. We were assigned the DOTSS 2 RNAV SID. As is typical at LAX; there was a short notice runway change which we dealt with fairly easily because we expected it. We were on the correct SID and accounted for all other aspects of the runway change efficiently. On initial climb; nearing 3;000 feet we were issued the following: 'Turn left heading 210 for traffic; climb and maintain 12;000.' The FO (First Officer) read back 16;000 and was immediately corrected. We set the proper altitude. We were then immediately handed off to the next controller. When we checked in we were cleared 'Turn left direct PEVEE; climb via SID.' We climbed to 12;000 and leveled off. At that time something felt wrong to me so I asked the FO to check with approach for higher. At that time the controller responded with 'You were given a climb via clearance. Confirm you acknowledged the climb via clearance.' We confirmed that we had; knew that we made a mistake and climbed onward to the top altitude of 17;000 feet.Busy airspace; reconfiguration of aircraft; unfamiliar clearance; continuous confusion due to extremely chaotic and dynamic ATC environment; unclear radio transmission from ATCClimb Via clearances should be removed from ATC procedures. They are seldom enough used that they present a significant threat to safety in and of themselves. Descend via clearances are very often used; but this is the first climb via that I have had in three and a half years. They serve the purposes of Air Traffic Control but can be confusing to pilots; especially during busy phases of flight and after having been cancelled and reinstated. At a minimum; the Controller should have to say 'Climb via SID; top altitude 17;000' to help better identify the clearance as a revised climb clearance and to help it 'stick out' in a pilot's mind during a busy phase of flight.Change our procedures to identify a climb via clearance differently than a normal clearance. So; instead of setting 17;000 in the MCP and the PM saying '17;000' the PM (Pilot Monitoring) should confirm the altitude in the MCP and say 'Climb Via top altitude 17;000.'


Second narrative: On the ground we were given clearance to climb via the Dotss 2 RNAV with a climb to 3k. Once airborne we were given a vector and a speed restriction with an eventual clearance to PVEE to 12;000 and 280 knots. We were then given a frequency change and the next controller gave us a climb via the Dotss 2 RNAV. As we leveled off it just did not feel right (i.e. it did not seem like we should be level); so we asked the controller for higher altitude. He immediately came back and said that we were cleared to climb via the Dotss 5 RNAV. So we started to immediately climb to 17;000. My best estimate is that we were level for a very short time.ATC gave us a climb via clearance; with all the other comms and crew coordination it just did not dawn on us that we were cleared to climb until we leveled off. It just did not feel right that we were level. So we checked with ATC who informed us that we were cleared to climb via; so we immediately started to climb.I think it would have helped us if the controller had given us the climb via clearance and maybe thrown in the top altitude to clue us in that we were cleared to climb again. Sometimes the climb via clearances seem to create more confusion than help.


Synopsis: B757-200 flight crew reported confusion relating to a Climb Via clearance.