This stems from the comments on this question. Imagine two cities, A and B, close enough that one can work the tiles that were acquired by the other (either through purchasing them or through cultural expansion). If a tile was acquired by A, and I manually select the tile to be worked by B, does the tile transfer "ownership" to B? So that if B were captured, the tile would go with it, or if A were captured, the tile would stay with B?
Working a tile does not seem to change the ownership of that tile. If the city is conquered, only the tiles the city directly acquired (culture or bought) are transferred to the conqueror.
Placing a city into an area already owned by the player will transfer ownership of the city tile and the initial inner ring of tiles to that new city.