The open category is the baseline level of the EASA framework for low-risk operations — no prior authorisation from the aviation authority is needed. This is where most hobby pilots fly, and many commercial operators with lighter drones as well.
To stay within the open category, strict limits apply: the drone's MTOM must be under 25 kg, the flight must be within visual line of sight (VLOS), the maximum height is 120 m, never over assemblies of people, no dangerous goods and no dropping of material. The category is split into subcategories A1, A2 and A3.