The first thing that you might notice is that OTRS::ITSM completely uses the agent and customer interfaces (customer frontend) implemented in OTRS. If OTRS has been used before, all features and steps such as login, queue configuration, user preferences, filters, rules, user permissions, etc. can continue to be used without any modifications.
The manual at hand will therefore only discuss differences between OTRS and new aspects of OTRS::ITSM, while paying particular attention to the following:
IT services and SLAs
New ticket fields and functions
ITIL compliant terminology
Detailed information on the settings and proceedings that are identical in both OTRS and OTRS::ITSM are provided at http://otrs.github.io/doc/manual/admin/5.0/en/html/ in the OTRS Admin Manual, which is continuously being revised.