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:
Boolean
Lists Boolean operators for evaluating Boolean expressions.
Converters
Lists converters for conversion between numerical and vector types.
Macro Tools
Lists Macro Tools with inputs and outputs for creating customized Macro Relations.
My Macros
Lists any existing Relations constraints. You can rename constraints in the Scene browser, and use the My Macros operator to store your custom Relations.
Number
Lists operators for evaluating mathematical expressions involving numbers.
Other
Lists miscellaneous operators, such as trigger controls and FCurve values.
Rotation
Lists operators used specifically for rotation values.
Shapes
Lists operators for use with models that have shape animations (also known as morph targets).
Sources
Lists operators for creating automatic movement, rotation, and shape animation by providing an oscillating data source.
System
Lists operators so you can use a time code and the play or record state in your Relations.
Time
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.
Vector
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.