The Object browser in the Relations pane appears with operators beneath the Constraint settings only when a Relations constraint is selected.
Relations constraint A. Object browser B. Relations pane
The Object browser lets you insert operators, devices, characters, materials, textures, and shaders that form a Relations constraint when connected.
The Object browser contains the following operators:
Lists Boolean operators for evaluating Boolean expressions.
Lists converters for conversion between numerical and vector types.
Lists Macro Tools with inputs and outputs for creating customized Macro Relations.
Lists any existing Relations constraints. You can rename constraints in the Scene browser, and use the My Macros operator to store your custom Relations.
Lists operators for evaluating mathematical expressions involving numbers.
Lists miscellaneous operators, such as trigger controls and FCurve values.
Lists operators used specifically for rotation values.
Lists operators for use with models that have shape animations (also known as morph targets).
Lists operators for creating automatic movement, rotation, and shape animation by providing an oscillating data source.
Lists operators so you can use a time code and the play or record state in your Relations.
Lists operators for comparing two different times expressed in either SMPTE time code or frames per second. These operators can be used, for example, to trigger rotations, translation, or animation based on the current time code.
Lists Vector operators for evaluating vector mathematics and other calculations involving vectors. All translation, rotation, and scaling values for models and assets are expressed as vectors.