The "work product doctrine" or "rule" was established with the U.S. Supreme Court's 1947 decision in Hickman v. Taylor, in which the court ruled that information or documentation accumulated or prepared on the part of a law firm in anticipation of potential litigation was protected from the phase of civil proceedings known as "discovery."
Discovery is a "where the wheels meet the road" process in which both parties to a civil suit disclose their documentation relevant to the proceedings. It is during the discovery phase that each side can see what the other has been holding back. Because the "work product doctrine" is vulnerable to abuse -- law firms can claim to have been working in preparation for civil litigation earlier than may have been the case, or a company that is a defendent in a civil case could, as a matter of routine, prepare legal documentation on a regular basis as a precautionary matter -- the doctrine is flexible relative to many aspects of the law. As explained by Supreme Court Justice Murphy wrote for the majority in Hickman v. Taylor:
"...petitioner emphasizes that the deposition-discovery portions of the Federal Rules of Civil Procedure are designed to enable the parties to discover the true facts and to compel their disclosure whereever theymay be found...and since the discovery provisions are to be applied as broadly and liberally as possible, the privilege limitation [i.e., the "doctrine"] must be restricted to its narrowest bounds."
And, in the same opinion:
"By endowing with immunity from disclosure all that a lawyer discovers in the course of his duties, it is said, the rights of individual litigants in such cases are drained of vitality and the lawsuit becomes more of a battle of deception than a search for truth...[However] Discovery...is not a one-way proposition...We agree, of course, that the deposition-discovery rules are to be accorded a broad and liberal treatmen. No longer can the time-honored cry of 'fishing expedition' serve to preclude a party from inquiring into the facts underlying his opponent's case."
In writing for the majority, Justice Murphy took pains to emphasize that the distinction between legitimate application of attorney-client privilege, the basis for the "work product doctrine," and the temptation on the part of lawyers and their clients to want to expand that category to unreasonable extremes was a tightrope that had to be crossed with caution.
Basically, in order for the doctrine to apply, the documentation in question need only fit into the criteria described above. The work product doctrine, as suggested, is not absolute. The side seeking documentation held by the side claiming its documents are protected under the doctrine can petition for the court for release of the papers in question. Such a petition has to make a strong case that the petitioner has good reason to believe that information vital to its case is being withheld unfairly, and that at least some of that information can be separated from more legitimately-protected documentation being withheld pursuant to the doctrine.