Digital voltage regulators now generally include more than just a simple voltage regulation module and what used to be an external manual control is generally built in the single device with addition of reactive power controller and power factor regulator.
In old systems an external voltage comparator will track both output of the Auto and manual channel and balance the Field Current Regulator (FCR) output to track and assure a bumpless transfer. In reality this was never perfect as the manual channel would not be on load and tricks had to be included to reduce the bump as much as possible.
In the digital AVR a function replaces this by monitoring each non-active regulation channel and updating the set points thus assuring bumpless transfer in any changeover between any modes. Additional flexibility is also added to allow for loss of sensing or unexpected changeover event by including a delay and a maximum rate of change in the tracker.
In some voltage regulators the exact same function will be adapted to allow for complete AVR redundancy where 2 AVRs will have a dedicated communication link to exchange their respective set point and providing bumpless transfer between the regulators. This function is called external tracking and added to the internal tracking provides full safety when changing modes or AVR without the concerns of disturbance on your system generation