Ichino–Ikeda–N. Harris conjecture for unitary groups

Let $\f$ and $\f'$ be factorizable vectors in representations $\c0$ and $\c0'$, respectively. Let $\a$ be the period pairing, ${\overline{\frak b}}$ the global arithmetic factor, \c the partial $L$-value, and $\d$ the normalized local integral at each place $v$ in a finite set $S$. Ichino–Ikeda–N. Harris conjecture. There is an integer ${\overline{\frak b}}$, depending on the $L$-packets containing $\c0$ and $\c0'$, such that

\a(\f,\f')=2^{{\overline{\frak b}}}{\overline{\frak b}}\c\prod_{v\in S}\d_v(\f_v,\f'_v).

This formula relates the square of a global period to a product of local periods and an automorphic $L$-value; the candidate is presented in the source as a conjectural Ichino–Ikeda formula, and no resolution is supplied in the provided text.