  Let $\mathcal C$ be a Krull-Schmidt triangulated category with shift functor
$[1]$ and $\mathcal R$ be a rigid subcategory of $\mathcal C$. We are concerned
with the mutation of two-term weak $\mathcal R[1]$-cluster tilting
subcategories. We show that any almost complete two-term weak $\mathcal
R[1]$-cluster tilting subcategory has exactly two completions. Then we apply
the results on relative cluster tilting subcategories to the domain of
$\tau$-tilting theory in functor categories and abelian categories.
