That`s a criteria that has to be stablished for each organization you work for. I had worked with companies that all their tasks, from their point of view are issues. Others where no issue is created. Everything is a task.
From my point of view, the simple definition is:
TASKS. Put everything you planned to do and all the changes during the way.
ISSUES. Put everuthying that is a obstacle to get tasks done. Or situations that may not be task related, but are an obstacle for the project.
For example, if a task called Test Software and you need a simulatio software to get it done. If for what ever reazon the software is not available, create an issue and have someone to take care of it. Or if the person supposed to do the task is on vacation, create an issue to have someone to make sure you have a new resource by the time the first one is gone. Or if there is a delay on a vendor, that will affect the hole project, you could create an issue to have someone start searching for a new one.
Regards¡
Raul Cardenas