Who this is for: Designers using the BOM Builder on the Design tab.
What it does: One click sends the finished BOM to the PO Manager. It uploads the BOM PDF to the project in Sunrise, updates the project's NetSuite Sales Order (racking and accessories), and checks that the Sales Order's modules, microinverters and batteries match the BOM. When the Purchase Order is cut later, it uses exactly what you finalized instead of reading the BOM back out of the PDF.
Until now, every PO was built by reading the BOM PDF in Sunrise. That broke in ways nobody could see from the PDF: BOMs printed through the wrong driver came through as unreadable text, a screenshot uploaded into the BOM category became "the BOM", and redesigns drifted away from what was ordered, so crews landed panels short. Finalize BOM skips the PDF reading entirely: the PO Manager gets the rows straight from the BOM Builder.
The button sits in the Bill of materials toolbar. It is greyed out until the BOM has rows, and while a finalize is already running.

S-3 Bill of materials: Finalize BOM is the orange button in the toolbar, after Print / PDF.
The dialog names the project you are about to update. Check it: Finalize writes to that project's Sales Order.

Finalize this BOM? names the project it will update in Sunrise and NetSuite.
A blue banner shows while the PO Manager uploads the BOM and updates the Sales Order. You can keep looking at the BOM, but do not click Finalize again or close the tab until the banner changes.

The blue Finalizing banner while the PO Manager uploads the BOM and syncs the Sales Order; Finalize BOM is greyed out.