You can find it in the filter menu and blur and field blur in Affinity Photo. You can also find a non destructive layer version in the layer menu and new live layer in the same location
Displays a panel with three options as well as a control handle to position the effect and you can control the strength as well as level of influence. You can also click the art board in different locations to add an additional pin (more than one) to again influence the effect. You can move the pins back and forth or select areas of interest in your image that you want to have in sharp focus and areas where you want the image to be dulled.
The global radius controls all the handles overall intensity and can be controlled via the panel as well as just dragging across the image which can extend the global value beyond the displayed 100px. You can modify the power and level of the handles for each handle in turn so select the handle and you will get a slightly thicker handle and the values will change. If you want to have a sharp object and the rest to be dulled, it seems, to me, work best with three different controls, one to the left of the object and one to the right and one on the object. The handles on the side have maximum level to obscure everything to the left and right and selected handle power is set to about 1.3 or so in all cases. The handle on the sharper object has a very low handle level. This is for one image, a different image with different objects may require a totally different setup of handles and settings. Sadly, they all lack presets in the current version (macros being a possible workaround for that)
You can always work on a layer, perhaps a new layer or a duplicate of the image as a layer and then use the effect. This gives the flexibility of being able to delete the effect as well as blending the effect with the underlying image. With the layer selected via the layers panel (view menu and studio) and apply the effect using as many handles or changes of settings as you wish (what works well with one image may fail in another). Once you have done that, you can then use the layers panel and the layer above and change the blending mode to difference or darken etc and the result often is a more dreamy look than just the default. You can also try this out in an easier way via the live layers
You can use it in different channels. Display the channels panel via the view studio menu and then select the red channel and apply the effect to that, repeat with the green channel but with different settings etc to create some extreme and unusual color effects.
It can be combined with all number of others such as the deform / twirl / perspective for distorting the effect as well as with add noise etc. Duplicate the image and then apply the effect as normal to the top selected layer and also then decrease the size of the layer using the handles. With the layer selected, go to the deform effect. You can create all kinds of ghostly weird distorted shapes from the combo of deform and the effect especially when using a gradient layer. The various steps can also be recorded into a macro and replayed at any point.
The effect has no presets, so he quickest and easier way to store the effect is the macro but as the handles depend on the image, the result may not be the same and the effect will modify the wrong part of the image. Still it can be used as an interesting visual effect such as combining a layer with a gradient and the deform and effect etc and this can then be generated at any point especially if the macro is saved to the library panel (view menu and studio and library). You can add multiple handles and set the handle level for each and have the handles close or handles far away and you can then manipulate the effect in many different ways with the colors of the shape smearing off / bleeding off the edge of the shape and those layers can be duplicated and combined using the layers panel and blending modes. You can also keep the shape live by using the live layer option in the layer menu
You can also use it with the artistic as well as frame text. Select the layer and then add the effect and then add the handles and the settings to obscure parts of the text and leave the rest fairly untouched by setting the handle value to close to zero. The blending modes are perhaps not as useful with this.
You can create shapes very quickly via the shape tools such as a donut tool or a cloud tool design. You can also select that layer and apply it to that layer. You can add multiple handles and set the handle level for each and have the handles close or handles far away and you can then manipulate the effect in many different ways with the colors of the shape smearing off / bleeding off the edge of the shape and those layers can be duplicated and combined using the layers panel and blending modes. You can also keep the shape live by using the live layer option in the layer menu