Patccc

affinity.help/photo/English.lproj/pages/SizeTransform/canvasSize.html
Note that Clip Canvas will have no effect if you have top level layers with "infinite" size, for example adjustment layers. I don't know if it's "by design", an oversight or a bug, but it doesn't even help if you disable them.
To clip the canvas to actual alpha>0 bounds, you'd have to – temporarily, if you wish – clip such infinite layers to a layer with "finite" size, i.e. a pixel layer or a (temporary) shape.

Welcome to the Serif Affinity Forums, @Patccc. 
‘Clip to Canvas’ (as per the topic title) clips the document so that only the objects in the canvas area are visible. ‘Clip Canvas/Unclip Canvas’ clips/unclips the canvas rather than the document (which is particularly useful for things like stock images, where the canvas is often too small initially).

Listen @oO5Dynasty, I don't know if you are really not getting the point, or if you are pretending not to. In any case, you are answers are really confusing and sometimes just plainly wrong. You just wrote that:
This is false. This whole topic is about the lack of possibility to do that in AP. We shown you even with videos what we mean by cropping to selection.
If you don't know the answer to something, then maybe, if you want to help people, you should consider the option of keeping quiet, instead of spreading misleading information.

@oO5Dynasty we all appreciate the fact that you are trying to help, so thank you for spending the time.
However, I think there must be some misunderstanding... "Crop to selection" is not about background removal and is not about masking. The only thing we care is to find the bounding box of a selection.
Here's an example of a picture cropped with GIMP. So, first here's the original picture:

Now here's how we crop with GIMP. Notice the step from having a selection to cropping with a single click...

Registrazione schermata 2020-04-28 alle 00.15.03.mov

It's alright, I wasn't being serious. Your idea to make a pixel selection, then use that to define the Affinity crop tool area sounds like a good one.

I know what you're talking about. I am aware of the non-destructive approach Serif chose for their cropping tools, which I think is quite useful. And while Adobe may use destructive cropping for crop to selection (I'm not positive), Serif doesn't have to implement the tool in the same manner.
 
Also,
 
- including this feature, even if destructive, would add functionality many users would care about (non-destructive cropping is nice, no doubt, but that's just icing on the cake. If the cake is missing, you don't want any icing?) 
 
- The sample code I included would approach cropping to selection in a non-destructive manner, since it basically uses the already existing crop tool and enhances its spectrum of application.
 
 
Not renaming, but defining a different custom input for the tool and give that input a name.
It could be implemented as a cropping mode ("To Selection"?) for the current cropping tool.
Writing the last selected cropping mode as preference to disk would be nice as well. So you don't always have to change modes when you use one a lot.
 
 
Exactly!

Such self-important arrogance. If this is the kind abusive, do-it-our-way-or-sod-off response people can expect when they come here for help, maybe I should stop recommending Affinity software.