Partner custom macros are site-wide macros that you can set on a per site/partner basis in your account.
Their main use case is to set up custom variables that are either network wide or site wide so you can easily pre-populate your monetization ad tags or prebidJS bidder setups with pre-made macro templates.
All of these macros are set on the site settings page and there is no limit to how many you can set per account.
Every partner macro is prefixed with _bp_ and then your custom value.
Once a partner macro is setup, these macros will be available to be used in different sections throughout the CMS like when adding a new ad source or prebidJS bidder.
The usual use case for these macros are an ID for a specific SSP bidder so you can easily set up header bidding, or an ad unit name for Google Ad Manager so you can easily populate a single ad source between multiple web properties.