Splunk ITSI is a premium Splunk application which is tailored to Splunk IT infrastructure monitoring.
It makes extensive use of the metrics store for performance monitoring, trending and service performance, KPI reporting and event (failure) prediction.
It has pre-defined workflows, dashboards and reports which are commonly found around IT service management environments, and is flexibly adaptable for specific environments.
It includes a number of glass-table views, visualizations, and panels (which are specific to the ITSI app) to help you visually identity, track and triage service affecting faults, often before they occur.
It is built on Splunk Enterprise , and it offers no new 'features' over and above the standard Splunk product, but a huge amount of 'heavy lifting' to implement industry relevant frameworks, searches, reports, workflows and a stack of searches is what you are paying for in the premium apps.