This field type is not mapped to any physical database field.
Virtual fields have no extra attributes.
You need to create a checkbox field for the user in order to select a record using a sequential workflow. But you don't want to save the value of the checkbox into the database. You would like to direct a workflow depending on the selection.
For this reason you would create a boolean virtual field Selected, which would be present in the data structure, but it would no persist to a database.