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.