Discovery Accelerator keeps a master collection of all items that have ever been accepted into any case. When a reviewer marks an item then, depending on how you defined that mark, the mark may apply just to the case or it may apply to the case and also be copied to the master collection so that it can be used in other cases.
For example, you may want a mark that is called Spam to stay with items in the master collection. If another search ever finds these items, they are already marked Spam and may not need to be reviewed again.
When you perform a new search and accept the results, Discovery Accelerator does the following for each item that you accept:
If you choose to keep the existing marks when you accept the search results, Discovery Accelerator adds existing retained marks to the case. If you do not choose to keep the existing marks, Discovery Accelerator removes them from the items in the case. However, it does not remove the existing marks from the items in the master collection.
When a reviewer adds to an item a mark that has the property "Items retain this mark for use in other cases", that mark stays with the item in the master collection so that it can be used in future cases. If the mark does not have this property, the mark applies within the case but is not available to other cases.