How Does the App Know Which Certificate to Supply?
    
    When using client certificates for authentication on macOS or Windows endpoints,
        GlobalProtect looks for a valid certificate meeting specific requirements and prompts the
        user to select the appropriate one if multiple certificates are available.
    When you configure GlobalProtect to use client certificates for authentication on macOS or
            Windows endpoints, GlobalProtect must present a valid client certificate to authenticate
            with the portal and gateways.
For a client certificate to be valid, it must meet the following
requirements:
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
                    won't 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's created by a certificate authority (CA). For more
                    information on specifying a common or custom OID, see 
Enable Certificate Selection Based on OID.
When only one client certificate meets the requirements above, the app automatically uses that
            client certificate for authentication. However, when multiple client certificates meet
            these requirements, GlobalProtect prompts the user to select the client certificate from
            a list of valid client certificates on the endpoint. While GlobalProtect requires users
            to select the client certificate only when they first connect, users might not know
            which certificate to select. In this case, we recommend you to narrow the list of
            available client certificates by certificate purpose (as indicated by the OID) and
            certificate store. For more information on these and other settings you can configure to
            customize your app, see 
Customize the
                GlobalProtect Agent.