Technically, it's z-k (z=0, k=15). It makes for a nice root change ID: zzzzzzzz. Less choking than kkkkkkkk.
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.)
Bookmarks take precedence over change ID prefixes. And indeed if you add a bookmark "xyz" and a change had a unique prefix "xyz", the change's unique prefix will grow by one. (the unique prefixes can already grow or shrink semi-arbitrarily so this doesn't "break" anything more)
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.)