Kelim, Chapter Twenty-Four, Mishnah Six
There are three different types of bases:
1) One which lies before a bed or before a scribe is susceptible to midras uncleanness;
2) One for a side-table is susceptible to corpse uncleanness;
3) And one for a cupboard is free from all uncleanness.
Section one: The base which is put in front of a bed or in front of a scribe is sometimes sat upon. Therefore it is susceptible to midras impurity.
Section two: The base next to a side-table is sometimes stood upon by a servant, therefore it is considered a vessel. But it is not sat upon therefore it is not susceptible to midras.
Section three: The one that serves as the base for a cupboard is not considered a vessel at all, and therefore it is not susceptible to uncleanness.