  We assume that stars may undergo surface differential rotation to study its
impact on the interpretation of $V\!\sin i$ and on the observed distribution
$\Phi(u)$ of ratios of true rotational velocities $u=V/V_\rm c$ ($V_\rm c$ is
the equatorial critical velocity). We discuss some phenomena affecting the
formation of spectral lines and their broadening, which can obliterate the
information carried by $V\!\sin i$ concerning the actual stellar rotation. We
studied the line broadening produced by several differential rotational laws,
but adopted Maunder's expression
$\Omega(\theta)=\Omega_o(1+\alpha\cos^2\theta)$ as an attempt to account for
all of these laws with the lowest possible number of free parameters. We
studied the effect of the differential rotation parameter $\alpha$ on the
measured $V\!\sin i$ parameter and on the distribution $\Phi(u)$ of ratios
$u=V/V_\rm c$. We conclude that the inferred $V\!\sin i$ is smaller than
implied by the actual equatorial linear rotation velocity $V_\rm eq$ if the
stars rotate with $\alpha<0$, but is larger if the stars have $\alpha>0$. For a
given $|\alpha|$ the deviations of $V\!\sin i$ are larger when $\alpha<0$. If
the studied Be stars have on average $\alpha<0$, the number of rotators with
$V_\rm eq\simeq0.9V_\rm c$ is larger than expected from the observed
distribution $\Phi(u)$; if these stars have on average $\alpha>0$, this number
is lower than expected. We discuss seven phenomena that contribute either to
narrow or broaden spectral lines, which blur the information on the rotation
carried by $V\!\sin i$ and, in particular, to decide whether the Be phenomenon
mostly rely on the critical rotation. We show that two-dimensional radiation
transfer calculations are needed in rapid rotators to diagnose the stellar
rotation more reliably.
