The function of holding brakes / motor brakes is to retain loads. (In contrast to control brakes or stopping brakes, which delay moving masses, and power brakes, which generate a counter torque for drive assemblies.)
Holding brakes / motor brakes are intended to stop shafts from starting up from standstill. Conventional motor brakes on servo axes are closed by spring force and ventilated via electromagnets: In the rest condition, springs or permanent magnets are used to push the moving braking surface against the stationary braking surface; they are generally disengaged automatically on start-up. Generally designed as a pure holding brake, the brake is only closed once the motor is at standstill. The motor actively brakes the load and the holding brake authorises emergency braking.
Holding brakes are used as safety brakes in lifting gear construction, elevator construction and mining. In general engineering they are used as an (emergency) stop and holding brake.