All sources of continuously measured or calculated data are called Sensors in the Robotics API. Such Sensors do not have to correspond directly to the physical devices that actually measure the data. The best way to think about Robotics API Sensors is to consider them pure sources of data, regardless of how this data is obtained exactly.
As examples, consider the encoders in the joints of a robot arm that read the current positions of the joints and the kinematic function of the robot arm which calculates the cartesian position from the measured joint positions. The Robotics API robot model provides Sensors (i.e. data sources) for both, though one is derived from the other and does not have a concrete physical equivalent.
Robotics API Sensors are realized by the package Sensor and its various classes. Instances of Sensors are in many cases provided by Robotics API Devices. For example, the abovementioned Sensors of robot arms are provided by the Devices Joint and RobotArm.
The following sections explain how to monitor Sensor values in programs, how to filter and combine Sensors and how to obtain States from Sensors for configuring and combining real-time operations (as explained in section 1.5).