KittyKilian

Are you a designer or programmer?  If the latter, you're in the wrong place.  You are here among VISUAL DESIGNERS.  That means we go with what we see, not with what theory or scientific data says to us.  If something looks sharper to my graphically trained eye (with Photoshop and Illustrator experience that spans the entire life of those products), then IT IS SHARP.
 
PERIOD.

justwilliam, I think that having a "Help" explanation open automatically would be great, it would be even better to be able to turn it off if you no longer needed it.

I wish that it would automatically switch methods based on whether you're shrinking or enlarging an image. Barring that, I wish that anything other than Nearest Neighbor was the default.

The help file of Affinity Photo should provide a guideline explaining the differences between these resample modes (the pros and cons).
 
Furthering the idea proposed by JDW, an interrogation mark should be provided directly in the Resample popup that, when clicked, makes appear an info box explaining these differences and —more importantly— proposes the best mode according to the app maker (Serif).
 
It's indeed peculiar that such basic information is nonexistent in the help file, in the resample popup, or on the web site of Serif!

Since no one knows what the difference is between Lanczos 3 "separable" and "non-separable" may I suggest that Serif put something more meaningful and descriptive into those parenthesis instead?
 
In my testing, Lanczos 3 (separable) enlargements and reductions a tad sharper than Bicubic, and Lanczos 3 (non-separable) is much sharper.
 
So I would think the Resample popup should read as follows:
Nearest Neighbor Bilinear Bicubic Lanczos 3 (sharp) Lanczos 3 (sharper) Because, quite honestly, "separable" and "non-separable" may appeal to the left-brainers, but isn't Affinity Photo appealing to our right-brain creative side?  Yes.  So make the descriptions more understandable, please!