You want to track newsletter signups to see them in reports, but you want the bidding algorithm to focus exclusively on optimizing for purchases. How should these conversion goals be configured?

Set purchase as account-default and newsletter as a secondary/observation goal. Account-default goals are the conversions that Smart Bidding optimises towards, so putting purchases there tells the algorithm exactly what to pursue. Secondary, or observation, goals are recorded and visible in reporting under 'All conversions' but are excluded from bidding, which is precisely the behaviour required: the team can still see how many newsletter signups campaigns produce without a single bid being influenced by them. Getting this wrong is a common and expensive mistake. If newsletter signups are left as a primary goal, they usually outnumber purchases by a wide margin and are far cheaper to generate, so bidding drifts towards traffic that signs up and never buys. Separating the conversions you measure from the conversions you optimise for is what keeps reporting rich and bidding focused.