Zones can contain several types of heat sources including, Heat pumps, Underfloor heating and Panels. The device priority defines in which order they are turned on and this setting is on by default. 

The device order can be changed as follows:

More Menu > Settings > Smart Features >  Change Order.

On this page you can change the order by dragging the sources into the order you desire. This is a system wide setting and applies to all zones and to only zones with more than one type of heat source.