Pressure treated Backing rail or roofing batten. 3.0m x 50mm x 25mm, used to fix capping rail too for closed board fencing. Only required when using notched posts and arris rails. Can also be used as roofing batten.
If you are using plain posts with flat rails there is no need to use a backing rail as the top rail can be used near the top of post and capping can be fixed to this instead.