A continuation (extension) line under a sustained figure shows the harmony holding while the bass note itself may repeat or tie

When one figure (or set of figures) applies across more than one bass note duration without change, a horizontal continuation line is drawn from the figure to the point where the harmony changes, rather than re-printing the identical figure at every note.

Identical figures are reprinted at every single bass note within a sustained harmony instead of being shown once with a continuation line — though Read and Gould both note reprinting every note is also a legitimate alternative convention, so this is advisory only.

Gould continuation lines; Read.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.