Make sure you understand Sharing Defaults for Populations and customizing Sharing Settings for Partners before diving into per-population overrides.
So you have set up your sharing defaults for your populations, which determine how your data is shared (or not) with all of your partner's populations. And you have learned how to customize these defaults with sharing settings for a specific partner, which will determine how your data is shared with all of that specific partner's populations. And now, you want to be able to share specific data (or not) with a specific partner's population. Enter per-population overrides. When we say "per-population", we are in the context of a specific partner, so what we really mean is "per-partner's-population".
Setting Per-Population Overrides
In your sharing settings modal, you will see an option on the bottom left to "Enable per-population overrides".
Note: this option is only available when your sharing setting is Overlap counts or Shared data. If your population is Hidden, your partner won't know it exists, and we don't support per-population overrides on Hidden populations.
Once enabled, a new section appears below:
Adding per-population overrides
1. Select your partner's population you want to create the override for in the dropdown.
2. Select which fields you want to share on the right and hit Save.
Note: these fields will be overlapped when your population (in this example All Accounts) overlaps with your partner's selected population (in this example Sabre's Paying Clients).
Viewing per-population overrides
You can view all of your overrides back on your Sharing Settings tab. They will appear under "Overrides" for your population you created them for.
You can also view overrides created by your partners with you in the Shared with You tab. In the example below, your partner (Sabre) created a per-population override for their Paying Clients so that data is shared when it overlaps with your Active Clients population. When Paying Clients overlap with any other of your populations, you will only see overlap counts.