This search will run the third_party_outages macro and pass the arguments EMEA and -24h to the macro definition. A search macro is a reusable chunk of SPL that can be inserted into other searches. A search macro can take arguments that are used to resolve the search string at execution time. The syntax for using a search macro is macro_name (argument1, argument2, ...).
Reference
See Use search macros in searches and Search macro examples in the Splunk Documentation.

Explanation
https://docs.splunk.com/Documentation/Splunk/8.0.2/Search/Abouttransactions In other cases, it's usually better to use the stats command, which performs more efficiently, especially in a distributed environment. Often there is a unique ID in the events and stats can be used.

Tags are aliases or alternative names for field values in Splunk. They can make your data more understandable by using common or descriptive terms instead of cryptic or technical terms. For example, you can tag a field value such as "200" with "OK" or "success" to indicate that it is a HTTP status code for a successful request.
Tags are case sensitive, meaning that "OK" and "ok" are different tags. Tags are created at search time, meaning that they are applied when you run a search on your data. Tags are searched by using the syntax tag::<tagname>, where <tagname> is the name of the tag you want to search for.

In Splunk, Pivot is a tool that allows you to report on a specific data set without using the Splunk Search Processing Language (SPL™)1. You can use a drag-and-drop interface to design and generate pivots that present different aspects of your data in the form of tables, charts, and other visualizations12.
One of the features of Pivot is that it allows you to save your reports1. This can be useful when you want to reuse a report or share it with others1. Therefore, it's not true that users cannot share visualizations created with Pivot or that they must use SPL to find events in a Pivot12. It's also not true that users cannot create visualizations with Pivot, as creating visualizations is one of the main functions of Pivot12.

Reference:
The fillnull command is a search command that replaces null values with a specified value or 0 if no value is specified. Null values are values that are missing, empty, or undefined in Splunk. The fillnull command can replace null values for all fields or for specific fields. The fillnull command can take an optional argument called value that specifies the value to replace null values with. If no value argument is specified, the fillnull command will replace null values with 0 by default.