A UAS operator is the legal or natural person who registers the drone activity and takes responsibility for the operation. The operator and the remote pilot are often the same person — but not necessarily: a company can be the operator while its employees fly the drones.
The operator's duties are concrete: register with the CAA, display the operator registration number on every drone they use, and ensure that remote pilots are competent for the relevant subcategory. This role is the centre of legal responsibility across the open category.