Plain English Meaning
Subpart I is the part of the federal aviation rules that tells airline operators how their airplanes must perform on takeoff, in the air, and on landing — for example, how much runway they need and how well they must be able to climb if an engine fails.
Full Definition
Subpart I is the section of 14 CFR Part 121 that contains airplane performance operating limitations for transport category aircraft used in air carrier operations, including takeoff, en route, and landing performance requirements such as runway length, climb gradients, and obstacle clearance.