פירוש על כלים 28:4

מצויירות טהורות – for their form proves upon them that they are designated for Scrolls alone, and not for the use of a person, and all that is not for usage of a person, is ritually pure.

Covers of scrolls, whether they are decorated or not, are susceptible to uncleanness according to the view of Bet Shammai. According to Bet Shammai, one might take a scroll covering and convert it for another usage. Therefore, it is susceptible to impurity.

ואינן מצוירות – designated also for the rest of the usages, therefore they are ritually impure. But the School of Shammai who [states] that they defile, whether they are painted [with figures] or not, because coverings for [Torah] Scrolls are considered for the usage of humans, that they are made to be used by them always.

Bet Hillel says: those that are decorated are pure, but those that are not decorated are susceptible. Bet Hillel says that this is true only of an undecorated scroll cover. It might be converted to other usage. However, a decorated scroll cover is not susceptible because one would take such a beautiful scroll cover and use it for some other need. Note that this accords with the mishnah in 24:14.

אלו ואלו טהורות – for since they were designated for [Torah] scrolls, it is further not for human usage. But the Halakha is not according to Rabban Gamaliel.

Rabban Gamaliel says: both the former and the latter are pure. Rabban Gamaliel says that even undecorated scroll covers will not be used for another purpose. Therefore they are pure.