Pro collects tax forms, gates payouts on them, and produces the payee list you need at year end. The free plugin does none of this — if you are paying US affiliates at any scale, that is the reason to upgrade.
Collecting the form
Affiliates complete a W-9 (US) or the appropriate W-8 (non-US) in their portal. The form is stored against the affiliate, and the affiliate can see what they submitted.
The payout gate
With the gate enabled, an affiliate without a completed form cannot be included in a batch. Their balance continues to accrue and remains visible to them, alongside the reason they cannot withdraw it — which is the difference between a gate and a mystery.
Year end
Pro produces a threshold-filtered payee list: affiliates paid at or above the reporting threshold in the calendar year, with the details their form provided. It is a list to hand your accountant, not a filing — the plugin does not submit anything to a tax authority on your behalf.
What this is not
It is not tax advice, and the thresholds are configurable because they change and because your obligations depend on where you and your affiliates are. Ask an accountant what you owe; use this to know who you paid.