I feel your pain, the TFS API documentation is terrible. However, here is my best attempt to guess what DeletionID is still.
When TFS deletes an element, it really just sets the DeletionID and stores the element around. Items that are not deleted have a DeletionID number 0.
Some methods accept a DeletionID to refer to items that were previously deleted, for example, if you want to restore them.
source share