The basic idea of the plot is to show simultaneously the relative size of estimated flows between regions. The origins and destinations of migrants are represented by the circle’s segments, where nearby regions are positioned close to each other. The size of the estimated flow is indicated by the width of the link at its bases and can be read using the tick marks (in millions) on the outside of the circle’s segments. The direction of the flow is encoded both by the origin colour and by the gap between link and circle segment at the destination.
As an example of such plots is the following one depicting migration flow between continents.
Migest relies on the package circlize. Looking at the documentation of circlize similar kind of graphics can be found, however they are not as beautiful and ploshed as the ones produced by migest.
I can definitely see many interesting applications to this kind of plots, as are evident in the many examples found scattered through the internet. Beyond depicting migration, this type of chart might be a valuable and beautiful tool to visualize any kind of flow between entities.