Moed Katan, Chapter 3, Mishnah 4. In this mishnah we continue learning what may be written and what may not be written on Chol HaMoed.
"Ein kotvin shtarei chovos ba'moed" - we do not write loan documents on the festival:
A loan document is a paper attesting that one person owes money to another. There is no necessity to write it specifically on Chol HaMoed: the debt does not have to be created now, and sometimes the parties trust one another and have no need for a document at all. The mishnah therefore rules that such documents are not written on the festival.
The mishnah does, however, list cases in which writing is permitted:
He does not trust him: when one of the parties does not rely on the other and demands that he hold a document.
The scribe has nothing to eat: when the writing provides the scribe with his livelihood.
In these two cases he may write, even on Chol HaMoed.
"Ein kotvin sefarim, tefillin u'mezuzos" - we do not write scrolls, tefillin or mezuzos:
Writing sacred texts in a professional manner - Torah scrolls, tefillin and mezuzos - is forbidden on Chol HaMoed. What is more, one may not even correct a single letter, and even the Sefer HaAzarah, the distinguished Torah scroll that was kept in the Azarah, which needed correcting, was not corrected on Chol HaMoed.
The view of Rabbi Yehudah:
Rabbi Yehudah distinguishes between writing for professional and commercial purposes and doing something for one's own needs, and he permits the following two:
Writing tefillin and mezuzos for himself: a person may write them for his own needs, provided that he does not sell them to others.
Spinning on his thigh: he may spin the wool for the tzitzis of his tallis, provided that he does so in a temporary manner and with a change from the norm - on his thigh, and not in his usual way.
This leniency too is conditional upon the act not being done for the commercial purpose of selling to others.
In summary: in this mishnah we learned that loan documents are not written on the festival unless the parties do not trust one another or the scribe has nothing to eat; that scrolls, tefillin and mezuzos are not written, and one may not even correct a single letter, even in the Sefer HaAzarah; and that according to Rabbi Yehudah, a person may write tefillin and mezuzos for himself and spin on his thigh for his tzitzis - as long as it is not done for the sake of sale and commerce.