  We review a notion of completeness in QFT arising from the analysis of basic
properties of the set of operator algebras attached to regions. In words, this
completeness asserts that the physical observable algebras produced by local
degrees of freedom are the maximal ones compatible with causality. We elaborate
on equivalent statements to this completeness principle such as the
non-existence of generalized symmetries and the uniqueness of the net of
algebras. We clarify that for non-complete theories, the existence of
generalized symmetries is unavoidable, and further, that they always come in
dual pairs with precisely the same ``size''. Moreover, the dual symmetries are
always broken together, be it explicitly or effectively. Finally, we comment on
several issues raised in recent literature, such as the relationship between
completeness and modular invariance, dense sets of charges, and absence of
generalized symmetries in the bulk of holographic theories.
