A MID Server needs to have access to the install.service-now.com domain to automatically stay up-to-date with instance versions. This domain hosts the MID Server upgrade files that are downloaded and installed by the MID Server when a new version is available. The MID Server checks for updates every 24 hours by default, or when the mid.upgrade.check_interval property is set12.
:
1: ServiceNow Discovery Documentation, MID Server Upgrade section
2: ServiceNow Support Article, Connections to allow list for the MID Server

The correct variable syntax to populate a custom column in a horizontal discovery pattern is $cmdb_ci_linux_server.u_custom_column.INSERT. This syntax indicates that the value of the custom column will be inserted into the cmdb_ci_linux_server table for the current CI. The other options are either invalid or incorrect syntaxes for this operation12.
References:
* Set Parameter Value operation
* Pattern variables

The WMI service is a Windows service that enables remote management of Windows devices using the WMI protocol. The WMI service is required for Discovery to access the configuration and status information of Windows devices using WMI queries. If the WMI service is not running on a host, it will prevent the discovery of Windows devices, as Discovery will not be able to communicate with the WMI Collector or execute WMI queries on the target device123.
References:
* How to resolve a failure to communicate with the WMI Collector during top-down discovery - Support and Troubleshooting - Now Support Portal
* Windows discovery without 'domain admin' or 'local admin' privileges ? - Support and Troubleshooting - Now Support Portal
* Discovery - ServiceNow Elite

The SNMP Query pattern operation executes an SNMP query on a target device and stores the results in a variable. The Variable Type field determines the format of the results. There are two valid Variable Types for this operation: Table and Scalar. Table is used when the query returns multiple values, such as a list of interfaces or processes. Scalar is used when the query returns a single value, such as the system name or uptime12.
References:
* SNMP Query pattern operation - Product Documentation: San Diego - Now Support Portal
* SNMP Query pattern operation - Product Documentation: Vancouver - ServiceNow