An indicator, an objective, and a window — in that order

Everyone wanted the site to be fast and nobody could say what fast meant, so the alert threshold was a round number somebody picked.

SLI  the measurement.
     "the proportion of requests to /checkout that return 2xx
      within 500ms"

SLO  the target for that measurement.
     "99.5%"

window  over what period.
     "a rolling 30 days"

all three, or it is not an objective. "the site should be up"
is none of them.

The order matters because the indicator is the hard part and the number is the easy one — a week spent choosing what to measure and ten minutes agreeing the target. Defining the window is what makes the objective checkable: 99.5% over a day and over a quarter are very different promises. Writing all three in one sentence is the artefact, and it belongs somewhere a non-engineer can read it.