The way in which WorldServer applies a translation memory (TM) is referred to as a mode.
WorldServer can apply translation memory (TM) to projects in the same way as does SDL Trados Studio 2009. In SDL WorldServer 10.0, users choose either the new Studio-aligned TM mode or the WorldServer 9.x TM mode when they create projects. Once that choice is made, it remains in force for the life of the project.
Whether you choose the Studio-aligned TM mode or the WorldServer 9.x TM mode depends on whether you want to integrate your projects with the SDL Studio translation supply-chain.