Reference for Niddah 6:3

כֹּל הַמִּטַּמֵּא מִדְרָס, מִטַּמֵּא טְמֵא מֵת. וְיֵשׁ שֶׁמִּטַּמֵּא טְמֵא מֵת וְאֵינוֹ מִטַּמֵּא מִדְרָס:

Anything that can be rendered impure with <i>midras</i> impurity [a type of impurity due to being sat on by certain types of impure individuals, rendering something an Origin of impurity] can be rendered impure with the impurity of a corpse. But there may be something that can be rendered impure with the impurity of a corpse but cannot be rendered impure with <i>midras</i> impurity.

Jastrow