https://www.juniper.net/documentation/us/en/software/junos/interfaces-security-devices/topics/topic-map/security-interface-logical.html

The default value of the dead peer detection (DPD) interval for an IPsec VPN tunnel is 5 seconds. DPD is a mechanism that enables the IPsec device to detect if the peer is still reachable or if the IPsec VPN tunnel is still active. The DPD interval determines how often the IPsec device sends DPD packets to the peer to check the status of the VPN tunnel. A value of 5 seconds is a common default, but the specific value can vary depending on the IPsec device and its configuration.
Reference:
Juniper Networks Technical Documentation: Configuring IPsec VPNs: https://www.juniper.net/documentation/en_US/junos/topics/task/configuration/ipsec-vpn-overview-srx-series.html

"Local Authentication-In local authentication, the SRX Series device validates the user credentials by checking them in the local database. In this method, the administrator handles change of password or resetting of forgotten password. Here, it requires that an user must remember a new password. This option is not much preferred from a security standpoint.
* External Authentication-In external authentication, you can allow the users to use the same user credentials they use when accessing other resources on the network. In many cases, user credentials are domain logon used for Active Directory or any other LDAP authorization system. This method simplifies user experience and improves the organization's security posture; because you can maintain the authorization system with the regular security policy used by your organization."
https://www.juniper.net/documentation/us/en/software/secure-connect/secure-connect-administrator-guide/topics/topic-map/secure-connect-getting-started.html

Global policies are used to define rules for traffic that is not associated with any particular zone. This type of policy is evaluated first, before any rules related to specific zones are evaluated.
For more detailed information about global policies, refer to the Juniper Networks Security Policy Overview guide, which can be found at https://www.juniper.net/documentation/en_US/junos/topics/reference/security-policy-overview.html. The guide provides an overview of the Juniper Networks security policy architecture, as well as detailed descriptions of the different types of policies and how they are evaluated.

User-defined security zones allow users to configure multiple security zones and share them between routing instances. This allows users to easily manage multiple security zones and their associated policies. For example, a user can create a security zone for corporate traffic, a security zone for guest traffic, and a security zone for public traffic, and then configure policies to control the flow of traffic between each of these security zones. Transit traffic can also be managed using user-defined security zones, as the policies applied to these zones will be applied to the transit traffic as well.