Rav Huna's explains that Rabbi Yehuda's pronouncement that the Alal has the laws of meat only if it was collected ["V'hu she'knaso"].
Rashi explains Rav Huna's statement as follows:
In collecting the Alal he has shown that it [the Alal] was not nullified initially ["dgalei daataei d'lo batlei me'ikara"].
Questions:
1. Why does Rashi say this? After all, even if the Alal was not important and was "batel" when the skin was flayed, a change occurred when the person collected the Alal - presumably to eat: in so doing he gave it the importance of food, similar to other Halachot where performing an act with an object gives it significance it did not have before, including issues of Tumah ["hashav alav"] and issues of issur veheter ["haticha naaseis nevela"].
2. In explaning "breira" often Rashi also explains that the choice shows RETROACTIVELY the the chosen portion was originally meant for this purpose. Why is this necessary? What indeed is the import of this explanation?
Yershayahu HaKohen Hollander
Rashi answers your question in the very next words. There is a difference between Tum'as Ochlim -- which requies an Ochel -- and Tum'as Neveilos -- which requires specifically Basar. Da'as helps to make skin into an Ochel, for an Ochel is subjective; whatever is eaten can be an Ochel. However, skin cannot be transformed into Basar for Tum'as Neveilah. Therefore, if at first it was not Basar the fact that later he is Kones it will not change skin into Basar.
D. Zupnik