Add to wallet is the moment of delivery: the button, link or QR code that puts a pass into a phone's wallet. On iPhone it hands over a signed PKPass file; on Android it opens a save link. Both platforms provide official button designs, and a good implementation detects the device so the member only ever sees the path that works for them.
The step is simple by design, one tap, no app store, no account creation on the phone. That simplicity is the core distribution advantage of wallet passes over apps: the wallet is already installed, so the entire funnel from "interested" to "carrying the pass" can be seconds long.
What matters more than the button is what stands before it. An add to wallet page that simply delivers a pass to someone who already exists in a system is distribution. A flow that creates a new member, captures consent and then ends in the pass is acquisition. The distinction decides whether the wallet channel grows a program or merely mirrors it, which is why hosted onboarding and post-purchase enrollment are the terms to press a vendor on.
Once the pass is added, the add to wallet moment is over and the lifecycle begins: updates, messages and identification at the till, for as long as the pass stays in the wallet.