Usually, they used for different purposes. The
addTaskLayer is used to add additional elements for the tasks. And because it usually takes the
task object as a parameter, it is more “flexible” in work with tasks objects
overlay extension is a more appropriate thing, if you want to add some additional elements to the whole gantt chart.
overlay has its own specific API methods, which are available only in this extension.
Here are some samples which could make the differences more understandable:
Thanks for the explanation. I think I got it better now.