Roaming improvement is done with the help of radio measurements.

Radio measurement helps clients or stations to investigate the radio environment to roam target AP efficiently so that the station can perform reliable roaming.

Clients or stations use 802.11k frames to measure the radio environment i.e. station request neighbor reports from connected AP.

In the enterprise domain, the standard radio measurement is done by the station by sending an action frame i.e Neighbor Report request frame and Neighbor Report response frame

Typically station sends a unicast Action Frame i.e Neighbor Report Request to the connected AP, requesting a Neighbor Report of the connected SSID.

Then AP responds to the station with a unicast Action Frame with a Neighbor Report Response with a list of neighboring APs, their BSSID, and operating channels.

Based on the BSSID Information subfield, the station identifies the roaming candidate

References :

Certified Wireless Analysis Professional-Chapter5