Commands that show a numbered list
/model and /theme both follow the same interaction pattern when run with no argument: a numbered list of options is printed, you type the number of the one you want, and pressing enter with nothing typed cancels without making any change.
This consistency is deliberate: once you have learned the pattern from using either one, the other behaves identically, with nothing new to learn.
Any future command that needs to present a list of choices to pick from would be expected to follow this same interaction, keeping the overall command surface predictable as it grows.
The numbers shown are positional for that specific listing, not a fixed ID tied to a particular model or theme; the same number can point to something different the next time the list is generated if the underlying set of options has changed.
This means it is worth reading the list each time rather than relying on remembering "option 3" from a previous session; the list itself is always the source of truth for what a given number currently refers to.