the stress level at which the pipe will fail/rupture or "break." The ultimate strength of the steel is determined by testing during the manufacture of the pipe.
The tension that a material can with stand without rupturing. Due to their high levels of ductility, most metals have a high value of ultimate strength.
Highest engineering stress developed in material before rupture. Normally, changes in area due to changing load and necking are disregarded in determining ultimate strength.