  The Slowly Pulsating B-type (SPB) stars are the upper main-sequence stars on
the HR diagram. Their oscillations are high-order, low-degree g-mode and can be
used to probe the structure of the radiative zone where is near the outer
boundary of the convective core and constrain the chemical mixing in stellar
interiors. In SPB stars, the period spacing periodically varies with periods.
It has regarded as a signature of the chemical composition gradient beyond the
convective core. Based on theoretical calculations, we find that the variation
frequency of the period spacings ($f_{\Delta P}$) is related to the width of
the $\mu$-gradient region on the buoyancy radius ($\Lambda_{\mu}$) with the
relation of $f_{\Delta P}\sim0.5\Lambda_{\mu}$. This indicates that the
variation frequency $f_{\Delta P}$ is sensitive to the central hydrogen mass
fraction $X_{\rm C}$ (i.e., the evolutionary status). Finally, we find that the
variation frequency $f_{\Delta P}$ and the means of the period spacings
$\langle\Delta P\rangle$ can be used to construct a new C-D-like diagram
($f_{\Delta P}$ vs. $\langle\Delta P\rangle$) which can be used to roughly
decide the stellar evolutionary stages and to approximately determine stellar
mass for SPB stars.
