This line appears only if you turn off the pattern scrolling while playing.
If you want to move it underneath the position you are, just hit enter and you will move it.
The “stop” line is for live processing reasons, when changing effect parameters, they are directly applied.
However, effect column parameters tied to effect plugins, you not always want to have these values instantly applied when you just changed the effectplugin assignment. You might not know what the original value was of the parameter that you changed of that new plugin assignment if you wanted to change a different parameter instead. That is why this “stop” line locks in place so that parameter changing does not occur in this event and you can safely change the DSP-effect number assignment and then afterward pick a parameter and value to change.
When pressing “Enter”, the line is processed and the “Stopline” is moved (if it wasn’t placed on that spot already)