If you are new to Lync/Skype for Business Server or don’t have expertise on Microsoft UC platform, rtc can create a big confusion. RTC, rtc and RTCLOCAL are either name of database instance name or database name. One simple way to remember it, generally lower case represents database name while upper case represents instance name. These names exist in both Standard and Enterprise Edition. To know more about other databases and structure see Skype for Business Server 2015 Databases
rtc: “rtc” is a database which store persistent users data such as contact list, schedule conferences and ACL’s etc. rtc is part of RTCLOCAL instance which exist on each Standard Edition and Enterprise Edition Front End Servers.
RTCLOCAL: “RTCLOCAL” is a database instance name which exist on each Lync/Skype for Business Server. When install any Lync/Skype for Business Server role SQL Express edition is used to install automatically.
RTC: “RTC” is one more database instance name which is used to install locally on Standard Edition Server and part of the SQL Express Edition. While Enterprise Edition Server uses SQL Server for Back End database, therefore RTC database instance is part of the Back End SQL database server. Make sure RTC is not a mandatory name for Back End Server instance but while deploying SQL server either you can use RTC or any other name for instance.