The link derives from a comparison between two classes of such objects: the classical T TAuri stars and the "naked" (or weak-line) T Tauri stars.
Among the distinguishing properties of classical T Tauri stars is an excess of infrared and ultraviolet emission compared to stars with similar surface temperatures and sizes.
Classical T Tauri stars are often surrounded by dust and gas.
"The odd thing is that it's been remarkably difficult to find spectroscopic binaries among the classical T Tauri stars."
That could explain why some naked T tauri stars have companions but no circumstellar material, whereas classical T Tauri stars, with their accompanying dust clouds, appear rarely to have companions.