One of the quiet bit of genius is that JJ change ids use a different set of characters for base16. Instead of 0-9A-F they use k-z
The relevance, btw, is that you can use git commit IDs in place of change IDs anywhere and they won't conflict; jj knows which you mean by the character set. (They could still conflict with bookmark names, sadly. But I haven't heard of that being an issue in practice.)