Keilim, Chapter 24, Mishnah 7. This chapter runs through one type of vessel after another and shows that a single name can cover three different objects: one that receives tum'at midras (the tumah of something made to be sat or leaned upon), one that receives only corpse tumah as an ordinary vessel, and one that receives no tumah at all. Here the Mishnah applies that same threefold division to the writing tablet.
First, a word about the object itself. A pinkas was made of two wooden boards fastened to one another with hinges, so that they closed together as a single unit. A person would write on such a tablet in more than one way: sometimes the surface was spread with sand and he would trace the letters in the sand, sometimes it held wax and he would etch into the wax, and sometimes he wrote directly on the wood with ink.
Our Mishnah opens with the words "shalosh pinkasiyot hen": writing tablets come in three varieties, and each one carries its own halachah.
The tablet spread with sand
"Ha'afiforin teme'ah midras" - the afiforin is a large tablet covered over with sand, on which a person would trace his words, and which was used mainly for working out calculations. Picture something like a blackboard, except that it did not hang on a wall; it lay down low, near the one using it. Because of its size and its position, a person could perfectly well sit on it while he was doing his figuring and writing on it. Since it serves as a seat at the very same time that it serves as a tablet, it is susceptible to tum'at midras.
The tablet with a place for wax
"Veshe'yesh bah beit kibul sha'avah teme'ah tmei met" - a tablet that has a receptacle for wax, where the wax sat and where a person would etch his letters into the wax so that the impressions remained in the shaped surface. Wooden implements become tamei when they have a beit kibul, a container, and this tablet has one for the wax. It is therefore an ordinary vessel in every respect and can become tamei with corpse tumah. But it does not receive tum'at midras, because one cannot sit on it while working on it.
The smooth tablet
"Vehachalakah tehorah miklum" - a smooth tablet is one with no beit kibul at all, without so much as a rim to hold wax or any coating. On such a tablet a person wrote with ink directly on the wood. It is pure from every form of tumah, for it is nothing more than a flat wooden utensil, and a flat wooden utensil does not become tamei.
So the same word, pinkas, covers three objects with three different halachot: the broad sand tablet that doubles as a seat and takes midras, the tablet with a receptacle for wax that takes corpse tumah as any vessel does, and the plain smooth board that remains tahor from all tumah.