Suppose the Haftorah is from Amos. Can one read from a klaf that is only Sefer Amos by itself? Or can one only read from a full set of Trei Asour in one scroll?
Trei Asar is one sefer that has 12 parts to it, and if it is missing the other 11 Neviim it isn’t complete. The benefit of reading from a klaf is only if it is a whole sefer and not if it is only a part. Therefore, it would be better to read from a whole nach (or even a whole Trei Asar) that is printed then from such a klaf. This scenario presents itself, if one wants to read Sefer Yona from a klaf on Yom Kippur, that it is better to read from a whole nach or at least a whole Trei Asar.
Baba Basra 14b, R’ Gershom ibid, Meiri (in shita Mekubetzes) ibid, Shulchan Aruch Y:D 283-1, Abarbenel in His introduction to Trei Asar (Hachakira Hateshi’is), Rama Mipano 20, Yerushalayim B’modaeha (Shabbos-2) pg. 189- 36. Also see Har Hamaor 1 pg. 51, who discusses this at length