The objective is a negotiation, not an aspiration

A number chosen by engineering alone is a number the business has not agreed to constrain the roadmap with, which is what an objective is for.

the conversation, with the person who owns the product:

  "at 99.9% you get 43 minutes of failure a month.
   at 99.99% you get 4, and it costs a second region,
   a redesign of the payment path, and a quarter.

   which do you want to buy?"

the answer was 99.9% for checkout and 99% for the
admin panel, which nobody would have guessed.

Presenting it as a purchase rather than a technical target is what makes the conversation productive, because the trade is genuinely between reliability and everything else. The admin panel at 99% is the answer that would never have come from engineering and is obviously correct once stated — nobody loses money when an internal tool is down for six hours a month.