Kelim, Chapter Twenty-Four, Mishnah Thirteen
There are three different types of sheets:
1) One used for lying upon is susceptible to midras uncleanness;
2) One used as a curtain is susceptible to corpse uncleanness;
3) And one used as a mural decoration is free from all uncleanness.
I think by now the principles underlying this mishnah should be clear.