The Talmud (Kiddushin 71b) discusses the differences in the lineage of people from different places in the vicinity of Bavel (Babylon) by analogy of health. Bavel is healthy (has a good lineage), Mishon is dead (totally bad lineage), Maday is sick (mostly good), and Elam is dying (mostly bad).
Immediately afterwards, the Talmud asks what the boundaries of Bavel are, beginning by quoting the opinions of Rav and Shemu'el.
My understanding is that this dispute about the boundaries of Bavel is about lineage, meaning which places have a good lineage. (The practical ramification could be that you can marry a Babylonian man or woman without checking, on the assumption that he or she has a good lineage.) This understanding seems to be shared by Rashi and is necessitated by the dispute between Rav Papa and Rav Yosef whether the boundaries under discussion can be applied to bills of divorce.
I have some problems with this.
Rabbi Yochanan is quoted later in the discussion about the boundaries of Bavel. However, the Gemara has already established that Rabbi Yochanan is unaware of the fact that Babylonians had a pure lineage. Rabbi Yochanan believes that there are also people with bad ancestry in Babylon. Why would Rabbi Yochanan discuss the boundaries of Bavel within the context of determining who has a good lineage, if he doesn't believe that Babylonians have an especially good lineage?
The Talmud Yerushalmi (4:1, 43a) also mentions this dispute between Rav and Shemu'el. While the Yerushalmi does have (quoting the Babylonians) the comparison between Bavel, Mishon, Maday and Elam, this is mentioned after the dispute about the boundaries of Bavel. The Yerushalmi gives these borders next to a dispute about whether to accept converts from Tadmor in marriage. Could it be that the dispute is about the border between Bavel and Tadmor, i.e. between Jewish converts from Tadmor who can't intermarry with Jews (according to one opinion) and Babylonians who can? And if so, would the Talmud Yerushalmi be interpreting this dispute differently than the Talmud Bavli, or could they both be sharing this understanding of the dispute?
Side point: It strikes me as strange that Rabbi Yochanan, from the land of Israel, voice an opinion specifically on the northern border of Babylon, and that the Yerushalmi is aware of Rav and Shemu'el's dispute (via Rabbi Yose berabbi Bon), but not Rabbi Yochanan's opinion. How can this be explained?