Publishers can control the initial bitrate, after that point the respective adaptive algorithm takes over and makes the decisions based on variables such as network utilization/congestion, screen size, geographic location and more.
Adaptive Streaming methods used by the Kaltura Players
With iOS devices Kaltura will leverage HTTP live streaming (aka HLS).
Where Flash is used, the choice will be adobe HTTP dynamic streaming (aka HDS).
Each respective algorithm will dynamically adjust the content stream on a set interval to fit available network bandwidth and device resource constraints.
Setting the initial bitrate will dictate what bitrate is used first.