When configuring the Discovery Online task, you can further specify how the task updates the relevant data.
Ticking the 'Update Empty Fields Only' option will mean that Discovery Online will only update fields where there is no data currently present. In most circumstances you will want this option ticked because otherwise it has potential to overwrite data you have entered yourself. If you have a resource with no 'Publication Date' for example, with this option ticked, this value will be updated. However, if you had manually added a publication date and have this option un-ticked then when the task runs it will overwrite the date you added.