Hello,
Task is a particular job assigned to a project member, or several members, or it can have no responsible person. A task can be subdivided into some subtasks. Is used in the
Projects module.
Milestone is an important stage in the progress of a project. Milestones can be used to schedule the workflow marking the moment when some significant result should be achieved.
To learn more information about how to work with the
Projects module, please, follow this link:
http://helpcenter.onlyoffice.com/gettingstarted/projects.aspx