This initial Collection would be a 'Standard' Collection, and only for placing together photos you want to initially take to the Print module.
First and foremost I create a saved print from the beginning ( printer icon appears) with the dimensions of the collage I want to print etc A3 or A2.
Nevertheles, from time to time I experience, that the pictures in the half finished or finished collage disappear even though they are still in the collection. Only the empty cells are left
If you [Create a Saved Print] "from the beginning", or "half finished", without photos in the cells, then that is what you will get- empty cells! in the 'Saved Print Collection'.
OTOH You have the option to create a Print Template Preset of your 'blank' collage layout. This will then always be available for any future use with any photos.
F_McLion suggested that a Print Collection can- "always can be reloadedbut not changed.". I disagree! It is quite possible from the Library Grid view to drag extra photos onto a 'Saved Print' Collection, and now you will see these also when you [double-click] the Print Collection to open in the Print module (select all on the filmstrip).
F_McLion also suggested that "You can update presets that you made by right-clicking" .
There is no [right-click] "Update Collection" menu item available for a Saved Print Collection, that I can see!, but if you change the layout of the Collage, it will automatically (in my experience) update the Print Collection template!
"saved in the catalog or anywhere else"
Always, and only, in the Catalog!- You 'preserve' by backing up the Catalog (as you should frequently!).