It refers to the required resistance to deformation or permanent damage due to loads applied at the car's ends, such as in a collision.
[1] Federal requirements for buff strength were set in 1999 at 800,000 pounds-force (3.6 MN) for all passenger-carrying units, unless reduced by waivers or special order.
The document was revised in 1992 and is presently known as VDV Recommendation 152 - Structural Requirements to Rail Vehicles for Public Mass Transit in Accordance with BOStrab.
In 1995 the European Common Market Committee for Standardization issued a draft document, Structural Requirements of Railway Vehicle Bodies.
[3] In general the European approach to crashworthiness of passenger coaches puts more emphasis on crumple zones rather than buff strength, meaning that the required design loads are less than those in the United States.