Diffuse or sharpen is a tool which can be used to either increase details or to add blur.
It differs from all the other tools above by the way it can be fine tuned to produce a greater and more appealing range of results, in turn it can be more resource consuming (in other words slow to perform).
Diffusion properties
The first three sliders are used in combination to define how much impact you want the effect to have (iterations) and how large the area affected will be (radius).
Diffusion
When diffusion sliders are moved to the right, blur will be added, while if you move them to the left sharpness will be increased. Diffusion speed, when reduced by a lot, resembles noise, while diffusion directionality is more like local local contrast.
Edges management
You can compare the effect of increasing edge sharpness to the high pass filter one.
Use with presets.
As this tool comes with a lot of options, I strongly advise you to play with its presets. Some included presets are specially made for denoising purposes (fine, medium, contrast), other presets are fast to compute (fast local contrast, fast sharpness), there are even presets to simulate a painting (watercolor) or a drawing (line).
When to use: Best used as the last touch of the post-processing, because otherwise it will slow down the whole editing process (due to its high processing power consumption).
After a denoiser, it is possible to retrieve lost details with the dehaze and other presets, without reintroducing much noise. Also consider using this tool with a painted layer as a way to highlight special features like hairs on a model, otherwise the effect might be too strong and impact too much of the picture.