What is Report ID used for in SSRS reports?
I assume that is a local .rdlc file.

IMHO it has not practical use, in fact you can copy &amp; paste as a new report using the same ReportID and it works.

Is it safe to create new .RDLC report as a copy of existing .RDLC report?

If you have a look at: RDL XML Schema for Version 2016/01 you won't find any reference to the &lt;ReportID&gt; tag, or if you get the RDL definition of any report stored in SSRS database, (Where does a published RDL file sit?), you won't be able to find this tag into the file definition.