A horizontal pattern must have a TCP entry point type or an ALL entry point type to execute from a process classifier. A process classifier is a rule that matches a process running on a host and triggers a horizontal pattern to discover the application associated with that process1. The entry point type determines the type of connection that the pattern uses to access the target host. A TCP entry point type means that the pattern uses a TCP port to connect to the host, while an ALL entry point type means that the pattern can use any available connection method2.
References:
* 1: Process classifiers - Product Documentation: San Diego - ServiceNow
* 2: Horizontal Pattern probe - Product Documentation: San Diego - ServiceNow

SNMP credentials are used by Discovery to communicate with devices that support the Simple Network Management Protocol (SNMP). SNMP credentials do not include a user name, just a password, called the community string. The default read-only community string for many SNMP devices is public, and Discovery will try that automatically. Enter the appropriate SNMP credentials if they differ from the public community string1.
1: SNMP credentials - Product Documentation: Utah - Now Support Portal