The gemara asks why we don't use bitul b'rov in the first case of the mishna when the animal that is assur b'hana'ah gets mixed with the others. The gemara answers that the animal is choshuv therefore too significant to be batul, either because the mishna holds that any davar sheb'minyan isn't batul, or because livestock is inherently significant and not batul. After the gemara establishes that the animal isn't batul b'rov, it continues to ask why we cant separate one by one from the group and bring them as korbanos. The gemara is understanding that after we establish that bitul b'rov doesn't apply to an important and significant item, we should still be permitted to separate one by one and bring them as korbanos each time relying on the statistical majority that the animal we are not sacrificing isn't the forbidden one. To that the gemara responds that it is considered "kavu'ah" so we aren't able to follow the rov. This seems to be a strange case of ka'vua! Normally kavuah applies to a situation where we have things that are established in their place such as kosher and non-kosher butcher shops, but it isn't clear how the concept of kavuah can apply here? The Chochmas Adam (binas adam, sha'ar kakavua 1) explains that the concept of kavuah on the torah level only applies to situations like stores where one buys meat from one of the stores and doesn't know if it was purchased from the 9 kosher or 1 treif. In that case the torah would consider it כמחצה על מחצה and we couldn't rely on rov. However, the kavua in our gemara is very different. In our gemara the gemara means to say that whenever chazal don't allow bitul to take place on an item due to it's significance, we treat the mixture as if it were kavuah. Meaning, not only do we disallow the concept of bitul b'rov on a significant item, but we treat the entire mixture as if is were kavuah and therefore don't even allow one to separate one by one and follow the statistical rov. The gemara continues to ask that we should be able to break the kavua status by sending the animals out and moving them from their established place, but even if we were to do that the Rabbonon were concerned that one may take an animal before moving them. It comes out that the Rabbonon create the concept of kavua on any mixture that contains an important issur on which we won't allow bitul. Once the Rabbonon consider it kavua, they make a second gezeira to be even more machmir than the kavua d'oraysa and we don't allow the kavuah to be broken by moving the animals around because they were concerned that one would take from it before doing that (Tosafos 73b d.h. ela, points out that from the fact that they make a gezeirah that you may come to take it before moving the animals while it is still kavuah, implies that if you were to take before moving the animals it would be d'oraysa. Tosafos rejects this and explains that even taking before moving them would only be d'raobonon). They weren't concerned about this by kavuah d'oryasa since the stores are recognizable in their place and there is no concern that if they permit the meat found in the street that one will come to permit meat that was bought from one of the stores.