How Does the App Know Which Certificate to Supply?

The certificate is issued by the certificate authority (CA) you defined in the Certificate Profile of your portal and gateway configurations.
The certificate specifies the client authentication purpose, which the certificate administrator specifies when creating the certificate.
The certificate is located in the certificate store, as configured in the GlobalProtect portal agent configuration. By default, the GlobalProtect app first looks for a valid certificate in the user store. If none exist, the app then looks in the machine store. If the GlobalProtect app locates a certificate in the user store, it will not look in the machine store because the user store takes precedence. To force the GlobalProtect app to look for certificates in only one certificate store, configure the
Client Certificate Store Lookup
option in the appropriate GlobalProtect portal agent configuration.
The certificate matches additional purposes specified in the GlobalProtect portal agent configuration. To specify an additional purpose, you must identify the object identifier (OID) for the certificate and configure the
Extended Key Usage OID
value in the appropriate GlobalProtect portal agent configuration. An OID is a numeric value that identifies the application or service for which to use a certificate and that is automatically attached to a certificate when it is created by a certificate authority (CA). For more information on specifying a common or custom OID, see Enable Certificate Selection Based on OID.