Roll Up Captions
The text scrolls up line by line. Each time a new line appears at the bottom, the top line disappears from the screen.
If you have received a caption file using "roll up" captions then you will need to perform a complex conform to retime the "roll up" captions to "pop on".
Simply changing the mode to "pop on" is insufficient as the timing of the events will be poor and the asset will definitely end up failing for timing issues.