The sentence

The promise itself, in final wording, treated as fixed.

The part

What it is

Written from the customer’s side and narrow enough that a specific failure is conceivable. Those two properties are what separate it from a slogan.

Length is a constraint rather than a preference. A promise that needs a second sentence is usually two promises with a priority nobody has set.

The smallest level

The attributes it carries

Each attribute is one of the framework’s shared types. What a date is, and how it is written, is defined once for the whole model — the note here says what it means in this particular place.

Promise
The exact wording, fixed so it is not paraphrased differently by each team.
Statement shared type
Language
A translated promise can commit to something slightly different; knowing the source settles disputes.
Choice shared type
Word count
A crude check — growth across versions usually means a second promise has attached itself.
Number shared type