The VigorAP-903 supports roaming that can reduce latency when wireless clients connect to another closer AP. This functionality is very useful for VoIP/Video and other applications.
The AP-assisted Client Roaming feature was depicted in this article https://www.draytek.com/support/knowledge-base/5764
Another feature to make roaming faster is PMK Caching and Pre-Authentication. Here is the reference article  https://www.draytek.com/support/knowledge-base/5619

Since 1.3.4 firmware, VigorAP903 is now compatible with 802.11r enabled clients. The benefits of 802.11r Fast Transition Roaming(FT) are exemplified below.

The new firmware brings an updated Roaming page:

How can 802.11r help with roaming?
802.11r uses Fast Basic Service Set Transition (FT). It allows encryption keys to be stored on all APs in a network. This way, the time for wireless authentication will be reduced. It means that a Wi-Fi client doesn’t need to perform the complete authentication process every time it roams to a new AP within the network range.

There are two FT mechanisms that are supported by Wi-Fi devices.
When a client moves from its current AP to a target AP using the FT protocols, the message exchanges are performed using one of the following two methods:

1. Over-the-DS

The client communicates with the target AP through the current AP. The communication between the client and the target AP is carried in FT action frames between the client and the current AP and is then sent through the controller.

 

2. Over the Air
The client communicates directly with the target AP using IEEE 802.11 authentication, and also the FT authentication algorithm.