The Shazzam Batch Size property determines how many IP addresses are scanned by a single Shazzam probe1. If this value is set to 1000 and a discovery must scan 10,000 IP addresses using a single MID Server, it creates 10 Shazzam probes with each probe scanning 1000 IP addresses. This property can be configured in the Discovery Properties module under Discovery Definition2.
1: Shazzam probe, port probes, and protocols - Now Support 2: [Discovery properties - ServiceNow]

The Discovery Dashboard provides a summary of the Discovery status, including the number of active discovery errors, the number of devices discovered, and the number of credentials used. Each error has a help link that provides more information about the cause and possible solutions.
:
Discovery Dashboard
Discovery troubleshooting | Error messages

The deletion strategy 'Mark as absent' means that when a related CI that was previously discovered by a pattern is no longer found, the install_status attribute of that CI is set to Absent. This indicates that the CI is missing or removed from the network. The main CI (in this case, the Linux Server CI) is not affected by the deletion strategy of the related CI (in this case, the Disk CI)12.
References
1: Set a deletion strategy - Product Documentation: San Diego - ServiceNow
2: Network adapter status setting to "Absent" in EMC ... - ServiceNow ...

A /24 subnet IP Range means that there are 256 possible IP addresses in the range, from 0 to 255. A Shazzam batch size of 5000 means that Discovery will send 5000 ICMP packets at a time to scan the IP addresses.
Therefore, Discovery will launch the Shazzam probe once for every 5000 IP addresses in the range, or 256 /
5000 = 0.0512 times. Since the number of times must be an integer, Discovery will round up to 1 and launch the Shazzam probe once. However, since the Shazzam probe is launched for each MID Server that is assigned to the Discovery Schedule, the actual number of times will depend on how many MID Servers are available.
If there are N MID Servers, then the Shazzam probe will be launched N times, each sending 5000 ICMP packets to the same IP range. Therefore, the total number of times that the Shazzam probe will be launched during discovery is N * 1 = N.
:
Discovery Schedule form
Shazzam Probe
Discovery IP range

For a pattern operation, the <_> placeholder can be replaced by any valid index or wildcard character that can be used to access an element of an array or a table. The $IfTable variable is a table that contains information about the network interfaces of a device1. The InstanceID attribute is a unique identifier for each interface2.
To access the InstanceID of a specific interface, the pattern operation can use the following syntax:
$IfTable[<index>].InstanceID
where <index> is either:
* A numeric index that corresponds to the position of the interface in the table, such as 3 for the third interface.
* A dot (.) followed by a key-value pair that matches a specific attribute and value of the interface, such as [.Name=eth0] for the interface with the name eth0.
* An asterisk () that represents a wildcard that matches any interface in the table, such as [] for all interfaces.
:
1: IfTable - Product Documentation: San Diego - ServiceNow
2: InstanceID - Product Documentation: San Diego - ServiceNow