Pattern regions are used when you want to active or switch a device's internal pattern from the timeline. Pattern regions are different to other regions in the way that they are not openable, meaning that they do not contain region markers or do not expose any kind of waveform or automation band. Pattern regions can only be created for devices which include internal sequencers.
Before a Pattern Region can be added to the timeline first a track for that region to live in must be created, this is done by going to the Add Track menu, selecting the device you want to make a pattern track for and selecting PatternTrack.
Once a pattern track has been created a pattern region can be created by double clicking anywhere inside the newly created timeline track.
As a default a newly created pattern region will always be for pattern A1 (pattern number 1 from pattern bank A). If you want to pattern region to play an internal pattern other than A1 it is possible to change the pattern by clicking once on the region to reveal the context menu and selecting switch pattern.