Wireless client roaming decisions are made by the client device based on its own criteria, such as signal strength, noise level, data rate, etc. The network can influence the client's roaming decision by providing information such as neighbor reports, load balancing, band steering, etc., but the final decision is up to the client.
References: https://www.arubanetworks.com/techdocs/Instant_86_WebHelp/Content/instant-ug/wlan- roaming/cl