A pass holder is a contact with a pass currently installed in a wallet. It is the honest measure of a wallet program's size: not how many members exist in the CRM, not how many passes were ever issued, but how many people are carrying the pass right now.
The distinction has teeth because passes have a lifecycle. A member can be enrolled but never add the pass; a pass can be added and later deleted. Wallet platforms know the difference: on Apple Wallet, a deleted pass unregisters its device registration, and Google Wallet reports pass state through its API. Pass holder counts therefore move, and watching them move is how a program learns what its members welcome and what drives them to the one-swipe delete.
Pass holders are also the messaging population. A lock screen campaign reaches pass holders, not CRM rows, which is why estimated reach is computed over them and why segments filter within them. A program with a large database and few pass holders has a mailing list, not a wallet channel.
Two useful derived metrics follow: install rate, the share of members who become pass holders, which measures how well enrollment flows work, and retention, the share who keep the pass over time, which measures whether the pass earns its place next to the payment cards.