Daf 21 speaks of issues related to infringing upon another's livelihood, and the differences that arise in varying situations. Are these prohibitions considered to be under the prohibition of Hasagat Gevul or are these prohibitions uniquely categorized under the seperate prohibition of being "Yored Leumnato"-infringing on one's livelihood?
The Gemara in Sanhedrin (81a) learns "Yored l'Umnaso Shel Chavero" from the Pasuk, "v'Es Eshes Rei'ehu Lo Timei" (Yechezkel 18:6). (See the Gemara in Makos 24a, which cites a different Limud.) However, the Rambam (Hilchos De'os 5:13) records this Halachah along with the proper conduct of a Talmid Chacham, and thus the Shulchan Aruch ha'Rav (Hilchos HefKer v'Hasagas Gevul 13) infers that it is only a Midas Chasidus.
Although the Halachos are diverse and complex, the basic premise is that if one is actually damaging the other and making him lose his investment (such as reprinting a Sefer), there is an Isur of Masig Gevul. In contrast, when entering into fair competition for future business (such as opening a competing store) it is classified as Yored l'Umenaso.