According to the Mishna, the measure that determines liability for carrying out parchment is equivalent to that which is used to write the shortest portion in the phylacteries. On today’s daf the Gemara discusses the status of parchment in contrast with another type of treated animal hide, which is called dokhsostos.
In the Talmudic era, the process of tanning hides for making parchment was very sophisticated. The desire to make optimal use of the hide and the goal of minimizing the thickness of books led to the manufacture of parchment that was especially thin. For that reason, the hide would be split. The parchment, the upper, more durable layer of the hide on which the hair grows, is called kelaf because they peeled away [kalfu] its inner layer. The less durable inner layer that faces the flesh is called dokhsostos.
The Rambam explains this differently than do most commentaries. In his opinion, dokhsostos is the upper part of the hide on which the animal’s hair grows, while parchment is the lower side. According to the Rambam, the Gemara’s comments on this issue define parchment as the part on the side of the flesh, while dokhsostos is the part on the side of the hair. However, the Gemara says nothing with regard to which side is used for writing (see Me’iri). Other commentaries hold that writing on both parchment and dokhsostos is done on the side where the two layers were attached and were separated from each other. They cite a verse as an allusion to that halakha, as it is written: “It is the glory of God to conceal a thing” (Mishlei 25:2). The Torah is written on the concealed part of the hide.
In practical legal terms, the difference between parchment and dokhsostos is based on the manner in which they are used. In order for the passages in the phylacteries to fit into their compartments, they must be as small and thin as possible. Therefore, they are written on thin parchment. Other texts, e.g., a mezuza, can be written on the less malleable dokhsostos (Me’iri).