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Customer browses products on mobile during lunch break. Returns home, uses desktop to complete purchase. Traditional cookie-based tracking sees two different users. Revenue attributes incorrectly or not at all. Cross-device tracking solves this by identifying the same person across devices using email, login, or platform-specific identifiers. Server-side tracking makes cross-device matching reliable by sending user identifiers platforms can match across sessions. The Cross-Device Attribution Problem Mobile session (12pm): User clicks Facebook ad on phone, browses products, adds to cart, doesn't purchase. Desktop session (7pm): Same user returns on desktop (different device, different browser, different cookie), completes purchase. Client-side tracking sees: Two unrelated sessions. Facebook doesn't get credit for driving the purchase. Your attribution is wrong. Server-side with email matching: WooCommerce captures email at checkout, server sends hashed email to Facebook CAPI and Google Enhanced Conversions. Platforms match email across devices, connect mobile browsing to desktop purchase. Attribution works. Email-Based Matching...