Custom Certificates are used as an encryption certificate to be provided by SP side to your Identity Providers.
You can use your own custom certificate instead of the general X509 certificate provided in the plugin by default. This adds another layer of security to your configuration.
Using your own custom certificate basically means replacing the default X509 certificate that comes with the plugin.
In order to use your own custom certificate, you’ll have to copy and paste your Public and Private keys here as shown in the figure below.