Open category and subcategories A1, A2 and A3
The default category
The Open category needs no prior authorisation, but it is bounded by hard limits: maximum 120 m from the closest point of the surface, visual line of sight (VLOS) at all times, no carriage of dangerous goods, nothing dropped, and no flight over assemblies of people. Within it there are three subcategories, matched to aircraft class and pilot competency.
A1, A2, A3
A1 uses C0 (under 250 g) and, with conditions, C1 aircraft; you may fly close to people but must not intentionally overfly uninvolved people. A2 uses the C2 class (under 4 kg) and lets you fly close to people but never over them: a minimum horizontal distance of 30 m from uninvolved people, reducible to 5 m only in the aircraft's low-speed mode. A2 requires the A2 Certificate of Competency - a 30-question multiple-choice exam, pass mark 75%, valid five years. A3 uses C3 or C4 aircraft and keeps you away from people entirely: no uninvolved person affected, and at least 150 m from residential, commercial, industrial and recreational areas.
Operator registration (Operator ID) applies to aircraft of 250 g or more, or any aircraft with a camera; the person flying needs a Flyer ID.
Traps: confusing A2's 30 m with an imaginary 50 m; believing the 5 m reduction applies with people still nearby (it does not - the area must be clear); thinking the A2 CofC is the same thing as a Flyer ID; assuming you may exceed 120 m because you own the land.
Rule of thumb - the 0-1-2-3 mnemonic: C0/C1 go to A1, C2 goes to A2, C3/C4 go to A3. Then remember three numbers: 120 up, 30 (or 5) out, 150 away.