A more thorough explanation:

The theory-of-pleading doctrine is an old principle that required a party to prove their case exactly as they pleaded it. This means that if a party did not include a specific claim or defense in their initial pleadings, they could not introduce it later in the case. However, modern codes and rules of civil procedure have abolished this strict requirement.

For example, under Federal Rule of Civil Procedure 15, a party can amend their pleadings to conform to the evidence presented during the case. This means that if a party discovers new evidence or wants to add a new claim or defense, they can do so without being bound by their initial pleadings.

One example of how this doctrine can affect a case is in a personal injury lawsuit. If a plaintiff initially pleads that they were injured in a car accident due to the defendant's negligence, but later discovers evidence that the defendant was also driving under the influence of drugs, they may want to add a claim for punitive damages. Under the theory-of-pleading doctrine, they would not be able to do so. However, under modern rules of civil procedure, they can amend their pleadings to include this claim.