I'm wondering what is exactly the difference between red lines and white lines in gridding?
Red lines are so called "baselines". They 'belong' to a certain object in the sense that they reflect, for example, where the boundary of an object is or where a source line is located exactly. In other words, those are lines whose position is given by the geometry, i.e. they cannot be shifted. They can be ignored (not used), but their position depends on the geometry and is fixed. One can specify in the grid settings how baselines are processed: The geometry resolution defines how close baselines can be to each other (to avoid overly refined grids). And because this leads to baselines that need to be ignored, there is the priority settings, which is used to decide which baselines to ignore. For example, to use all available baselines for an important object, set the resolution low and the priority high...
The white lines on the other hand are normal grid lines that are put in between baselines. The grid engine tries to match the maximum step and the global settings (padding etc...) as best as it can by filling in normal grid lines between the baselines.
Thank you for the great explanation.