A Discovery schedule of type Configuration Item collects complete information from the scanned IP ranges and sends it to the CMDB. This type of schedule runs a series of probes and sensors to identify and classify the devices and applications on the network, and to create or update the corresponding configuration items in the CMDB. A Discovery schedule of type Configuration Item can also run patterns to discover more details and relationships about the configuration items.
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ServiceNow Discovery Overview
Create a Discovery schedule
Discovery schedule types

One method for deleting specific CIs not discovered in 30 days is to use a scheduled job that runs a script to query the CMDB for CIs that have not been updated by Discovery for a certain period of time, and then delete them or mark them as retired. This method can help you keep your CMDB clean and accurate, and avoid having stale or obsolete CIs. You can configure the scheduled job to run at a specific frequency and time, and specify the criteria for selecting the CIs to delete.
References: Automated way to disable/retire CIs not updated for 45 days, ServiceNow CIS-Discovery Sample Questions

Horizontal discovery patterns are a series of operations that tell Discovery which CIs to find on your network, what credentials to use, and what tables to populate in the CMDB. Horizontal discovery patterns use variables to store and pass information between the operations. By default, there are two variables that are automatically available for horizontal discovery patterns: infrastructure_system and computer_system. These variables store the information about the infrastructure and the computer system of the target CI, respectively. They are populated by the Horizontal Pattern probe and sensor, which enable Discovery to use patterns for horizontal discovery123.
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Patterns and horizontal discovery - Product Documentation: Tokyo - Now Support Portal Horizontal Pattern probe - Product Documentation: San Diego - Now Support Portal Using patterns for horizontal discovery - Product Documentation: Vancouver - ServiceNow Reference: https://docs.servicenow.com/bundle/quebec-it-operations-management/page/product/service- mapping/reference/r_PatternVariables.html

The Credential Affinity table is a table that associates an IP address and a credential in Discovery1. It is used to store the credential that was successfully used to access a device during Discovery2. It can also be used to manually assign a credential to an IP address or a range of IP addresses3. The other options are not tables that associate an IP address and a credential in Discovery. The Service Affinity table is a table that associates a business service and a credential. The Service CI Association table is a table that associates a business service and a configuration item (CI). The Tags table is a table that stores tags that can be applied to records in any table.
1: Credential Affinity table 2: Credential affinity 3: Manually assign a credential to an IP address : Service Affinity table : Service CI Association table : Tags table