What is a “vacuous matrix”?

I am currently learning about robotic grasping and came across a matrix $$H_i \in \mathbb{R} ^{l_i \times 6}$$ which selects $$l_i$$ components of the relative contact twists and sets them to zero. My issue is that the rotational component of $$H_i$$ is said to be "vacuous" for the hard finger and point without friction models. Could someone please explain what the vacuous means in this context? Is it just a zero matrix?

• i have that in my head some days ... lol – jsotola Jul 31 at 2:16