Via-Points and NoGo-Areas
BRouter can process via-points and nogo-areas, and brouter-web offers on online interface to edit both of them.
Nogo areas are useful if you encouter an obstacle that is not contained in the underlying map. But they can also be useful in planning alternative routes by excluding e.g. a certain bridge or ferry.
For offline use, nogo-areas can be defined as wayoints using a special naming convention. Handling of via-points during offline use depends on the mode of operation, see the README for details.