  In this work, we focus on the period-luminosity relation (PLR) of $\delta$
Sct stars, in which mode excitation and selection mechanisms are still poorly
constrained, and whose structure and oscillations are affected by rotation. We
review the PLRs in the recent literature, and add a new inference from a large
sample of $\delta$ Sct. We highlight the difficulty in identifying the
fundamental mode and show that rotation-induced surface effects can impact the
measured luminosities, explaining the broadening of the PLR. We derive a tight
relation between the low-order large separation and the fundamental radial mode
frequency (F0) that holds for rotating stars, thus paving the way towards mode
identification. We show that the PLRs we obtain for different samples are
compatible with each other and with the recent literature, and with most
observed $\delta$ Sct stars when taking rotation effects into account. We also
find that the highest-amplitude peak in the frequency spectrum corresponds to
the fundamental mode in most $\delta$ Sct, thus shedding some light on their
elusive mode selection mechanism.
