How to determine whether command should be control or meta?

Question

Is there a set of criteria to determine whether a command should be a ctrl keybinding or a meta keybinding?

For example, file handling commands seem to fall under C-x bindings. Cursor movements are a mix of ctrl and meta. I don't notice much of a pattern other than built-in bindings leaning towards ctrl which suggets meta may be somewhat reserved for user bindings.

Answer 1

I'm not aware of a fixed set of criteria to distinguish between commands bound to Ctrl or Meta. However, the Emacs Lisp manual has a section on Key Binding Conventions that should be interesting to you.

Meta is not officially reserved for user-defined bindings:

Sequences consisting of C-c and a letter (either upper or lower case) are reserved for users; they are the only sequences reserved for users, [...].

If you're asking because you want to set up custom key bindings for commands that don't have any by default, I suggest you also take a look at Emacs: How to choose good custom key bindings? and Are there any emacs key combinations reserved for custom commands?

Additionally, when binding keys be aware that

A <Control>-modified alphabetical character is always considered case-insensitive: Emacs always treats C-A as C-a, C-B as C-b, and so forth. [...]

For all other modifiers, you can make the modified alphabetical characters case-sensitive when you customize Emacs. For instance, you could make M-a and M-A run different commands.

... as described here.