Whether it works · The control · Element
The cost
What holding back the control costs.
The part
What it is
Whatever the personalisation is worth, multiplied by the holdout share. Usually small and worth stating so the decision is deliberate.
Where the holdout costs nothing, the personalisation is producing nothing, which is itself the answer.
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.