The diffuse filter for Affinity Photo can be found in the noise category of the filters menu. It can be used multiple times or once and can be combined in millions of ways with others.
There is only radius. You can use the slider to push it up to 100% but you can also use the interactive option by dragging across the image from left to right or up and down. Even if the value says 100%, the interactive option can push the noise beyond that.
It can be used with images and type as well as with brush strokes and fonts designs. You can also use it with shapes, vector shapes that are created by the shape tools or perhaps shapes created with the pen tool and manipulated by the node tool. You can select some text and apply the destructive effect but you can also add a live layer as well (via layer menu) and the effect can be changed / edited at any point, you can change the underlying design as well as the text itself and also add a mask using something like a gradient to show more or less of the grain.
You can repeat apply the effect either with the same setting or by changing the settings (there is only one) you can get a very different effect added. The only trouble with using the repeat is that you have to keep the effect fairly low first time around and then apply it repeatedly. If you go for extreme noise, repeat applies result in just more and more noise. You can still combine it with the layer menu and fade and use with blending modes and opacity to change the result.
You can also combine the effect with others and if you wish you can record the various steps via the macro panel and replay it on other images. You can combine it effectively with many different features in the application such as Gaussian blur as well as box blur and deform and many others. You can then blend the effects using the fade command.
You can also record the steps via the macro panel and play them back over and over to create all kinds of unique designs from any image, you can use the effect with subtle settings and apply 30 times with different blends to create very unique effects.
You can also use it with the powerful fade command found in the layer menu, the fade allows you to blend the result with different blending modes such as difference, lighter color, overlay and more. You can also modify the opacity of the effect / original image
You can use it with layers, especially powerful if you reduce the size of the newly duplicated layer and then apply the effect. You can then take the result of that and distort it in 1000s of ways with the powerful mirror and deform. You can change the noise levels and as the same time you can go to the layers panel and change the blending mode for that layer. You can also go to a different layer and change the effect for that
You can use channels, just go to the channels panel found in the view menu and studio. You can then select the red channel and apply it to that with one set of settings and then go to the blue channel etc with a different set as well as combine with others.
Selections are also super useful as you can apply it to a selected area created via the select menu or via the selection tools such as freehand or rectangle marquee. You can then move the selection and then apply a different effect on that.
It can also be used in combo with the powerful pattern layer from selection, so you can create some truly awesome patterns as well as again combine with others in the distort menu etc. If you scale down the image and then apply effect then a high setting will totally blast the image, it is probably a good idea to reduce the value of the intensity setting.
You can also use it via the layer menu and the live layers, it is in the same category of noise. It can be edited quickly to change the intensity as well as the blending mode of the layer by double clicking the entry in the layers panel. The effect is also applied to the entire image or layers below or it is applied to one layer only. You can also combine it with color options via the adjustment layers
You can use the layer menu and new adjustments layers such as HSL or vibrance etc to re-color the underlying noise effect so instead of just having a basic colorful noise, you can change it to different colors. You can also add a solid or gradient etc layer above and re-color the effect by using different blending modes with the color layer.