The subset of the feature's geometry within the screen extent will be used to calculate the position of the label. The label is more likely to be visible on-screen, if there is room. However, the label may move position as the user pans the screen and different parts of the feature geometry are used to calculate the label position.
The feature's entire geometry will be used to calculate the position of the label. This may result in a label not being visible because the preferred location is outside the extent. For example, a line label placed at the start or end of the line might be off-screen if the start or end of the feature geometry are off-screen.