  We identify and examine the solar wind intervals near the sonic critical
point (i.e., $M_S \sim 1$) observed by the Parker Solar Probe (PSP). The near
subsonic wind intervals show similar properties: a low density, an extremely
low velocity, a low proton temperature, and essentially no magnetic field
deflections compared with the surrounding solar wind. The extremely low
velocity is the primary contributor to the near crossing of the sonic critical
point rather than the sound speed, which is roughly constant in these
intervals. Source tracing with a potential field source surface (PFSS) model
suggests that the near subsonic intervals all connect to the boundaries inside
coronal holes. Heliospheric current sheet (HCS) and partial HCS crossings
around the near subsonic intervals indicate that the near subsonic wind is a
transition layer between the slow and fast wind. The above scenario is
consistent with the nature of the near subsonic wind as a low Mach-number
boundary layer (LMBL), which facilitates the crossing of the sonic critical
point at 15-20 $R_S$. Moreover, we find a dependence of the amplitude of
switchbacks on the radial sonic Mach number. Magnetic field deflections
essentially disappear near the sonic critical point, which suggests that
switchbacks originate from above the sonic critical point.
