Efficacy signal
An efficacy signal is what a user picks up from a button, ad, link, or other element that tells them what they're going to get if they engage with it. The label on the button, the image in the ad, where the link sits on the page, how the copy reads: all of this feeds the user's anticipation of what's on the other side. Users pick between options based on those signals, not based on direct knowledge of what each option will actually deliver.
Definition
What the user reads in encountering an affordance — the basis on which they anticipate what the affordance offers and what comes from engaging it.
What signal includes
Anything the user perceives in the encounter that contributes to their anticipation of what the affordance will deliver: visual indications, textual content, positional context, behavioral cues, copy phrasing. A button's label, color, placement, and the headline above it are all signal; so are the platform conventions the user has built up over years of similar interfaces. Signal is multi-modal — the user reads several channels at once and composes an anticipation from them.
Selection is signal-mediated
When the user faces a set of reachable affordances, the option whose signal best fits the route they're on gets preferred. This makes selection prediction-mediated: the user commits to an option based on what its signal anticipates, not on direct knowledge of what the option will actually deliver. Direct knowledge would require having already engaged the option, and at the moment of selection the user hasn't. Signal is the proxy. A well-fitting signal makes the right selection feel obvious; a poorly-fitting one forces interpretive work or routes the user toward something else.
Continuity across steps
Signal carries motivation forward across hopscotch — the consistency between what one affordance signals and what the next affordance offers is what allows the user to maintain motivation through pursuit. When the next affordance contradicts the prior signal, motivation doesn't just fail to extend; it retroactively unmakes, since the basis for the prior selection turns out to have been misleading. This is the scent-breaking failure mode.
Example
A user landing on a SaaS homepage sees two buttons: "Start free trial" and "Talk to sales." They don't know what either click actually produces — only what the labels suggest. They pick based on signal: the first signals self-serve, no commitment, instant access; the second signals enterprise, contact-first, sales process. The user on a quick-evaluation pursuit clicks the first; the user evaluating a procurement decision clicks the second. Both selections are predictions made from signal, not from knowledge of what the next page will show.
If "Start free trial" routes to a sales contact form, the prior signal turns out to have been misleading and the user's motivation for the click dissolves — they wouldn't have clicked had they known. The failure is between the two affordances: the prior signal committed to one thing, the next contradicted it.