For the game, every feature which is not completed and there is no meeple are unoccupied. Doesn't matter, if that feature is unoccupied from first moment, when tile with feature were placed and player not placed there a meeple, or it was Abbot removed during game, or it was Dragon which eat meeple or meeple was prisoned by The Tower expansion and there is other options how can be meeple removed during game from unfinished feature.
In the moment when feature is unoccupied and unfinished you can plece there a meeple via arrow:
Instead of placing your meeple on the tile just placed, you can place it on any unoccupied, incomplete feature (a road, a city, a field, a monastery, or even an abbot on a monastery or garden) anywhere in the playing area - so it doesn't have to be on the tile just placed. Please note that fields completely surrounded are not considered completed, since they are not scored until the end of the game.