Page hint or sticky note.

[pioneer]

Have the programmers considered allowing for a non printable page hint for how you have done something within the document (such as in PagePlus) or sticky note to all of the 3 programs.  I for one would find this extremely useful.

Or an area such as the Assets where you could store notes?

@walt.farrell suggested a workaround but I could not use that in Affinity Photo

Cheers John