Text from MicroStation idea MSR-I-1420 for this:

Along the same lines as another idea I posted for floods 'seeing' visible edges, I would also like to be able to define an element to NOT be seen by flood operations.

My other idea: MSR-I-1419

This would allow users to define elements to NOT be considered by a flood operation, just like dimension lines are ignored, such as a centerline drawn through an assembly section, or the level for a grid drawn for cross sections (more of an ORD function, but built in the MSTN engine). Just like being able to select an element and choosing the properties to make it non-snappable, or setting a leve to non-plotting, etc.