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While studying FAA's Standard Instrument Departure Procedures, I came across the La Guardia Five Departure SID. Take, for example, the Take-Off Minimums for runway 13L:

Rwy 13 (Coney Climb): 400-2 or Standard with minimum climb of 280' per NM to 500. ATC climb of 900' per NM to 1500.

How should I interpret this "ATC" indication? What difference does it have from the previous minimum climb gradient requirement ("climb of 280' per NM to 500") which omits this particular indication?

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I believe I have found the answer to my own question, while examining the records of a meeting held by FAA concerning this subject ( source ). It clearly states:

[ AERONAUTICAL CHARTING FORUM Instrument Procedures GroupMeeting 17- 02 –October 24, 2017 ] When ATC crossing altitude restrictions require a climb gradient on a SID, it is proposed to establish a single climb gradient that would allow compliance with all ATC restrictions. These climb gradients would be identified on the chart as ATC climb gradients. Rich explained how the aircrew should treat each climb gradient differently. An obstacle climb gradient should be treated as a plane that the aircraft cannot ever penetrate from above, that is, the aircraft must always remain above the climb gradient’s surface. Conversely, an ATC climb gradient only represents the average gradient needed to meet the crossing restriction at the fix where it’s established. In this case, the aircraft’s altitude between the point where the climb starts and the point where the altitude restriction is established is not that important, provided the crossing is made at the fix at (or above) the minimum charted altitude. The purpose of the ATC climb gradient is to inform the pilot of the necessary performance needed to comply with the altitude restriction. The working group suggested that a SID should be limited to one obstacle climb gradient, and if necessary, one ATC climb gradient. Further, an ATC climb gradient and the associated crossing restrictions should be eligible for cancellation by ATC when not needed or when aircraft performance is limited.

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