From SKYbrary Wiki
Nimbostratus is a Mid or Low level, rain bearing layer cloud.
Generally covering the whole sky, and thick enough to hide the sun, Ns is dark grey in appearance and with a distinctively diffuse base because of falling rain or snow.
Stratus Fractus and Cumulofractus
There may be detached fragments of cloud beneath the base of the Ns which are known as Stratus Fractus or Cumulofractus.
Pilots flying under visual rules should avoid flying beneath Ns because of the reduced visibility in Rain or Snow and the Stratus Fractus. Flight through or below Ns may be associated with downdrafts or Turbulence, but not as severe as would be encountered with Cumulonimbus (Cb). However, if the temperature is below 0°C, nimbostratus may be associated with In-Flight Icing.