Figured bass hold lines indicate that chords remain the same over changing notes in the bass.
By default, Dorico Pro shows hold lines for figures that have duration, but hides lines between suspensions and resolutions or after resolutions. You can hide/show hold and suspension lines for individual suspension figures. You can also change the circumstances in which Dorico Pro automatically shows hold lines, and their position and number, project-wide on the Figured Bass page in Engraving Options.
Tip
You can move figures graphically in Engrave mode, including changing the graphical length of hold lines.