Sub 250 Gram drones
You see a lot of drones advertised as "Sub 250 gram" or "249 gram" or "Under .55 pounds" like the:
dji Mini,
dji Mini SE,
dji Mini2,
dji Mini 3,
dji Mini 3 Pro,
dji Mini 4,
Autel Evo Nano,
Autel Evo Nano Plus,
Hubsan Zino Mini Pro, and
Holy Stone HS510.
Air NEO (1.98 oz; 56.13 grams)
AirPIC (1.92 oz; 54.43 grams)
HOVERAir X1 (125 grams)
This weight class only affects three things: 1) Drone Registration, 2) Remote ID Broadcast, and 3) OOP or Operation (flying) Over People. Let's look at each of these.
Drone Registration
Part 107 does not make any distinction on the weight of a drone, so ANY drone flown under Part 107 rules must be registered.
For Recreational Flyers operating under the exception for limited recreational operations of unmanned aircraft, look at the back of your The Recreational UAS Safety Test (TRUST) Completion Certificate and you will see that you are required to "Register your drone if it weighs more than 0.55 pounds". That's 0.55 pounds at takeoff and throughout the flight. If you add any payload (such as an anti-collision light (strobe), prop guards, or any other payload that causes the takeoff weight to be over 0.55 pounds, then you need to register the drone.
Remote ID Broadcast
The requirement to broadcast Remote ID is directly tied to drone registration. If you need to register your drone, you need to broadcast Remote ID. If you don't need to register your drone, then you don't need to broadcast Remote ID ... unless your drone came with standard (built-in by the manufacturer) Remote ID.
§ 89.110 Operation of standard remote identification unmanned aircraft.(b)(3)
OOP - Operation over People
Subpart D—Operations Over Human Beings
§ 107.110 Category 1 operations, states that (a) A remote pilot in command must use a small unmanned aircraft that— (1) Weighs 0.55 pounds or less on takeoff and throughout the duration of each operation under Category 1, including everything that is on board or otherwise attached to the aircraft; and (2) Does not contain any exposed rotating parts that would lacerate human skin upon impact with a human being.
For most "sub 250" drones, by the time you add the prop guards (required for 2), you are over the weight limit (required by 1). Fortunately, under Part 107 you can apply for a waiver of § 107.39 Operation over human beings.
See: § 107.205 List of regulations subject to waiver.
What are the rules for sub-250 gram drones?