The remote pilot in command is the person §107.19 puts in charge of a small unmanned aircraft flight. That person must be designated before or during the flight, is directly responsible for and the final authority as to the operation, and must be able to direct the aircraft so the flight complies with the rules. Section 107.12 is what requires that person to hold a Remote Pilot Certificate and meet §107.65, except for the narrow foreign-authorization case in §107.12(c).
Designation before or during the flight
Section 107.19(a) is a timing rule. Designate the remote pilot before launch, or designate during the flight if the person in command changes while the aircraft is airborne. Both are inside the rule. A reading that allows designation only before the flight, and forbids a change during the flight, adds a prohibition the text does not contain. A reading that allows the name to be chosen within 24 hours after landing misses the timing in the other direction. After the flight is too late. The requirement also does not depend on whether a visual observer is used. A flight with a visual observer still has a remote pilot in command. A flight without one still has one.
On a two-pilot survey, the morning pilot is designated before takeoff. At the battery change, the afternoon pilot is designated during the operation, before that pilot's flight. The log can record the handoff as it happens. It cannot invent the morning pilot after the aircraft is already on the ground and a question has been asked about who was responsible.
Directly responsible, and the final authority
Section 107.19(b) is the authority sentence. The remote pilot in command is directly responsible for, and is the final authority as to, the operation of the small unmanned aircraft system. The owner does not keep final authority while the remote pilot "advises." Responsibility is not limited to the minutes flown by hand. An automated segment is still the operation. The remote pilot does not step out of §107.19(b) because a mission plan is running, and §107.19(e) still requires the ability to direct the aircraft during that automated flight.
The crew can help. Help is not a split of final authority. Assigning traffic scan to a visual observer does not make the observer the final authority. A contract that says the client directs the aircraft does not rewrite §107.19(b). If the operation cannot be done safely, the remote pilot is the person who stops it. How an uncertificated person may still touch the controls, without becoming the remote pilot, is on the supervision page.
The ability to direct the aircraft
Section 107.19(e) requires the remote pilot in command to have the ability to direct the small unmanned aircraft to ensure compliance with the applicable provisions. "Direct" means the pilot can make the aircraft comply: change course, stop a bad automation path, or land. It is not a 30-second landing timer. The regulation does not say the aircraft must be on the ground within 30 seconds of any command. It is not a requirement to keep a telemetry feed up for its own sake. It is not a requirement to be in continuous contact with air traffic control. Those are familiar aviation habits, and they are the wrong answer to this paragraph.
A workable example is a grid mission toward a cloud layer. The remote pilot sees the aircraft will come within 500 feet vertically of the cloud, which §107.51(d) does not allow. The ability §107.19(e) requires is the ability to direct the aircraft away from that path, or to end the flight, so the operation stays inside the rule. A telemetry number on the screen, with no way to change the path, does not meet the paragraph. A radio call to approach control does not meet it either, unless that call is somehow what directs this aircraft. The pilot directs the aircraft.
What people add to the rule
The traps add a clock, a boss, or a gadget. The clock trap says designation is only before launch, or only after landing, or only when no visual observer is present. The text says before or during the flight. The authority trap says the owner, the client, or the software is final, and the remote pilot advises or covers only manual flight. Section 107.19(b) says the remote pilot is directly responsible and is the final authority for the operation. The gadget trap replaces §107.19(e) with a 30-second landing, a telemetry stream, or an open ATC frequency. The paragraph is the ability to direct the aircraft so the flight complies.
In an in-flight emergency that requires immediate action, §107.21 lets the remote pilot deviate from any rule of Part 107 to the extent necessary to meet that emergency, and a written report is due if the Administrator requests it. That emergency authority sits on top of §107.19. It does not erase the duty to have been designated, or the duty to be able to direct the aircraft. The deviation rule is the in-flight emergency page.
Work the designation and authority items on the free practice test. The lessons that teach §107.19 are at pricing.