All objects have some required properties and some that are optional. As indicated in the discussion of objects above, objects are monitored and controlled only through their properties. BACnet specifies 123 properties of objects. BACnet also may require that certain objects support specific additional properties. The type of object and the type of device in which that object resides determine which properties are present.
Some properties can accept writes, and others can only be read.
When a property is read or written to, that act is known as a service. The only service that is required to be supported by all devices is the Read-property service. There are a total of 32 standard services. As a system developer or user, you don't need to be concerned with the execution or processing of service requests, which will be transparent and automatic. As a specifier or engineer, however, you will need to know what objects and services are supported by which devices.