How a portfolio works
You create the strategy once, from Bid strategies under Budgets and bidding in the Tools menu, or while setting up or editing a campaign, and then add campaigns to it. The campaigns bid toward the same goal and target, and you change settings for all of them in one place in the Shared library. Google lists the Smart Bidding strategies Maximize conversions, Maximize conversion value, Target ROAS and Target CPA, plus Maximize clicks and Target impression share. A portfolio's strategy type can't be changed later, for example from Target CPA to Target ROAS, so you create a new one instead. Portfolios used to be called flexible bid strategies.
Bid limits
Portfolios are where bid limits live. Google's Target CPA help page says max and min CPC bid limits are only available for portfolio, not standard, Target CPA strategies, are used in Search Network auctions only, and aren't recommended because they restrict Google's bid optimization. For portfolio Maximize conversions and Maximize conversion value strategies, the Google Ads API reference says bid limits are disregarded on Search campaigns.
Portfolio bid strategy vs shared budget
A portfolio pools the bidding. A shared budget pools the money. You can use either one alone, and Google calls it best practice to implement shared budgets with portfolio bidding and lets you link the two. Smart Bidding portfolios can also be managed across client accounts from a manager account. In our experience, a portfolio helps most when several low-volume campaigns share one goal and one target, and it's the wrong tool for campaigns whose conversions are worth different amounts to you.
Common questions
Can Performance Max use a portfolio bid strategy?
No. Google's help center says portfolio bid strategies are not available for Performance Max campaigns.
Which bid strategies can be portfolios?
Maximize conversions, Maximize conversion value, Target CPA and Target ROAS, plus the automated strategies Maximize clicks and Target impression share.
Can I set a maximum CPC bid in a portfolio bid strategy?
For portfolio Target CPA, yes. Google says max and min bid limits are portfolio-only and used in Search Network auctions only, and it doesn't recommend setting them.
Can I change a portfolio from Target CPA to Target ROAS?
No. Google says an existing portfolio's bid strategy type can't be modified. Move the campaigns to a different portfolio or create a new one of the type you want.
Is a portfolio bid strategy the same as a shared budget?
No. A portfolio shares one bid strategy and target across campaigns, and a shared budget shares one daily budget. Google recommends using them together and lets you link them.