What are non-pixel elements

[denjes2]

I have jpegs that I want to edit and I just want to save them as flattened. Sometimes I get this dialogue box that there are "The document contains non-pixel elements. Would you like to save the document flattened to pixels, or save as a new document?" And other times I don't get the box. Jpeg's are from same set of RAW files. I basically crop and apply curves. I merge the curve adjustment. And sometimes, applying same set of edits, I can either save and it saves directly, or it comes up with this dialogue box. Seems random. It does seem, though, that it occurs more often when I crop twice before saving, or cropping and them applying curves. 

[Old Bruce]

Non-pixel equals Text or other Vector objects. I may also refer to an adjustment layer or a Live filter. Cropping is non destructive in Affinity Photo so the bit you cut off is still there.