The %m teg was used to define scrolling margins.
%s%m|25|151| Disaster is coming.
This would display "Disaster is coming." beginning at 25 pixels on the left and cut it off at 151 pixels on the right and then start scrolling. This tag is obsolote because you can do scrolling with viewports now.
To get the same results, you simply have to create a viewport with similar dimensions.
%sDisaster is coming.
The y position of the viewport is arbitrary. You have to try out some values, to see, if the line with the %m tag is on the same level as the viewport.