Kelim, Chapter Twenty-Four, Mishnah Ten
There are three different types of mats:
1) One used for sitting upon is susceptible to midras uncleanness;
2) One used by dyers is susceptible to corpse uncleanness.
3) And one used in wine-presses is free from all uncleanness.
Section one: A mat used to sit upon is obviously susceptible to midras uncleanness.
Section two: The mat used by dyers is not sat upon (would be a bit messy). But it is considered a vessel so it is susceptible to corpse uncleanness.
Section three: The mat used in wine-presses is used to cover the grapes. Therefore, it is not a vessel and it is not susceptible to uncleanness.