Why the setting matters
Smart Bidding optimizes toward every primary action in the goals a campaign uses. Google groups actions into goals by category, so two primary actions in the same category get optimized together, and every account-default goal applies to new campaigns automatically. If a light action like a signup and a heavy one like a demo are both primary with the same value, the bidding treats them as equal and tends to find more of the cheaper one.
Where each one shows up in reports
Primary actions in goals the campaign uses fill the Conversions and Conversion value columns. Everything else, secondary actions and primary actions in goals the campaign doesn't use, shows only in All conversions and All conversion value. That is why the Conversions column can look smaller than the activity you see in the account: it only counts what the bidding is pointed at.
The custom goal exception
Custom goals let you combine any mix of primary and secondary actions, and Google says a secondary action included in a custom goal is used for bidding when a campaign uses that goal. A campaign can bid toward only one custom goal. Google calls custom goals an advanced option it doesn't recommend in most cases.
How to use it
Make the action you actually want the only primary action, and set the rest to secondary so you keep the reporting without paying the bidding to chase them. Google gives the same advice: if several goals track stages of one funnel, make only one of them the account default. If several actions matter, give each a conversion value that reflects its worth and use value-based bidding. Changing a campaign's goals restarts Smart Bidding's learning phase, which Google puts at 7 to 14 days. The full walkthrough is in our piece on why Smart Bidding can't tell a demo from a signup.
Common questions
Does a secondary conversion action affect bidding?
Not normally. Secondary actions are excluded from the Conversions column used for bidding and reported in All conversions. The exception is a custom goal: a secondary action included in a custom goal is used for bidding when a campaign uses that goal.
What two conditions make a conversion action count for bidding?
It must be set as primary, and the campaign must use the conversion goal that contains it.
Why does my primary conversion action not show in the Conversions column?
Because the campaign doesn't use the goal that contains it. A primary action only counts in Conversions, and for bidding, in campaigns that optimize toward its goal. Elsewhere it shows in All conversions.
Should I have more than one primary conversion action?
Only if each one matters and they carry values that reflect their worth, with a value-based bid strategy. Otherwise keep one primary action per funnel and set the rest to secondary, which matches Google's advice to make only one funnel stage the account default.
Does changing primary and secondary actions reset learning?
Changing the conversion goals a campaign bids toward restarts Smart Bidding's learning phase, which Google puts at 7 to 14 days. Make the change once and give it time before judging results.