You can define your retargeting audiences with a single click when you create or edit your Bottom of Funnel audiences, or you can create a custom retargeting audience.
Customized Retargeting Audiences
If you would like to retarget with conditions that are not listed in the three default options, scroll to the bottom of the options and click the text:
"Do you want to make an advanced selection?"
From the new window you can specify the Behaviour (View, Add to Cart, Purchase), Product Set, and the Time Frame (people within the last 20, 50, 100 days, etc.)
If you want to make additional selections and increase the audience, click "and also" to make additional selections.
If you want to make exclusions and decrease the size of the retargeting audience, click the text that says "Create exclusion".
Adding exclusions is an important step to ensure that you are not retargeting the people who have already made their purchase (unless of course you want to target past purchasers).
The additional selections and exclusions windows also allow you to specify the Behaviour, Product Set, and Time Frame.