While most university style guides mention "List of Figures", "List of Plates", "List of Tables", I haven't found a concrete definition of what might be included as a Figure or Plate in a thesis. Majority of theses seem to use Figures to label everything that is not a table but I wanted to know if I can separate image cutouts, code snippets and photographs e.t.c. into Figures or Plates properly. Thing is, I just don't have a clear guideline on where each might fit in these two types of listings.
I would be surprised to see anything called a plate if it wasn’t printed on a different kind of paper from the rest of the text.
More specifically, every “plate” I have encountered has been an image printed on glossy paper to improve its appearance and allow for more color and detail, when the main text has been printed with black ink only on rougher paper.
My understanding was that figures include maps, graphs, diagrams, and plates are reserved for photos or illustrations.
Though plates are less used these days, instead using figures for everything as you say. Code snippets if short can be included in text, or presented as you would a formula (indented), without a caption.
Code should not appear in either, it should appear in a separate "list of listings." Similarly, you could put photographs in a separate "list of photographs." You can introduce as many categories as you need, but be conservative.
For LaTeX, you can define custom floats (e.g., for photographs) and you can list all such floats. In some cases (e.g., for code), custom environments already exist, use those.