Section 107.3 defines a small unmanned aircraft as an unmanned aircraft weighing less than 55 pounds on takeoff, including everything that is on board or otherwise attached to the aircraft. That definition is the door into Part 107. Two tighter numbers still apply after you are inside. The manufacturer publishes a maximum takeoff weight for the specific aircraft, and you do not exceed it. Category 1 operations over people use 0.55 pounds, again including everything attached.
Less than 55 pounds, as it will take off
Weigh the aircraft the way it will leave the ground. The camera, the strobe, the guards, the payload, and anything else attached are part of the takeoff weight. An airframe that is under 55 pounds before the payload goes on can leave the small unmanned aircraft definition once the payload is installed. Part 107 is written for the aircraft that is under that line. Fifty-five pounds even is not "less than 55 pounds."
Registration does not follow a lighter cutoff for Part 107. An aircraft flown under this part is registered even when it is well under 55 pounds, including aircraft under 0.55 pounds. The recreational exception that can leave a very light aircraft unregistered does not apply here. Marking and the $5, 3-year registration are on the aircraft registration page.
The manufacturer's number is the one you fly
The maximum takeoff weight of a given drone is set by the manufacturer, and it should not be exceeded. Air traffic control does not publish that number for your airframe. The remote pilot's preference does not either. A short flight is not a reason to go over it.
When the manufacturer's performance table lists a maximum takeoff mass, and lists reduced endurance for the temperature and altitude you will actually fly, use that row. If the aircraft plus payload exceeds the listed mass, remove payload or choose another aircraft and another plan that stay inside the data. Do not fly carefully on the theory that the job is short. Do not copy the endurance figure from a lower altitude or a cooler day and pretend it applies. Exceeding a published mass limit, or extrapolating past the table, removes the margin the table was built to protect.
Mass is not the only loading limit. An aircraft can be under its maximum weight and still be outside the approved center of gravity, which changes controllability. That balance question is on the center of gravity page. Staying under 55 pounds, and under the manufacturer's mass, does not answer it.
Category 1 counts the accessories
Category 1 is a different ceiling, used for a specific kind of flight over people, and it is easy to miss because the accessories feel like safety equipment. A drone with a bare airframe weight of 240 grams does not stay in Category 1 after the pilot adds a 30-gram strobe and a 20-gram set of prop guards. Takeoff weight including everything attached is 290 grams. That exceeds 0.55 pounds (250 grams). The aircraft does not qualify for Category 1 on that flight.
Safety accessories are not excluded from the addition. The manufacturer's listed airframe weight is not the only number that counts. Guards and strobes are allowed on a Category 1 flight only when the total stays at or under 0.55 pounds and no exposed rotating parts would lacerate human skin. The full Category 1 rule, including the Remote ID condition for sustained flight over an open-air assembly, is on the Category 1 page.
Three limits, three wrong answers
A test item about "the Part 107 weight limit" is the §107.3 line: less than 55 pounds on takeoff, everything attached included. A test item about whose number you must not exceed on this airframe is the manufacturer's maximum takeoff weight, read from the performance data for the real temperature and altitude. A test item that adds a strobe and prop guards to a 240-gram airframe is the Category 1 arithmetic: 240 plus 30 plus 20 equals 290 grams, which is over 250 grams.
The traps swap those ceilings. Pilots exclude the guards because they are "safety gear," or they fly over the table because the site is close, or they treat 55 pounds as a target instead of a line the aircraft must stay under. Weigh the configuration you will launch. If the table says the mass is too high, change the load. If the Category 1 total crosses 0.55 pounds, that aircraft is not a Category 1 aircraft on that flight.
Check the weight items in the free practice test. Loading and the small-UAS definition are in the course at pricing.