Work-Product Doctrine
This privileges documents prepared by a party or their attorney in anticipation of litigation. The litigation does not have to be already started.
Ordinary work product
documents, information, facts, witness statements
Opinion work product
legal opinions, mental impressions, how the evidence relates to/is relevant to the litigation
When can work product be discoverable?
Note: opinion work product is almost never discoverable, so this rule only tends to apply to ordinary work product.