MISHNAH. IF HE DEPOSITED [THE ERUV MEAL] ON A TREE ABOVE [A HEIGHT] OF TEN HANDBREADTHS, HIS ERUV IS INEFFECTIVE; [IF HE DEPOSITED IT AT AN ALTITUDE] BELOW TEN HANDBREADTHS HIS ERUV IS EFFECTIVE.
The discussion ensues about why this would be the case. A private domain has no height limit (rises up to the sky). And if was put in a basket and suspended from a tree, there is no height limit. At least, according to Rabbi; so long as the tree is four handbreadths in width. But the Sages disagree – because while the eruv may be effective, the suspended food is not permitted to be moved on Shabbat, making it – in effect – ineffective!
It is this rather that he meant: [If the pole was] ten [handbreadths] high it is necessary that at its top it shall be four [handbreadths wide], but if it was not ten [handbreadths] high it is not necessary for its top to be four [handbreadths wide].
This is part of the argument, found on Shabbat 5a and 101a, in which a basket suspended on a pole in a public domain cannot have anyone toss something into it from a private domain.