With Identity Suite 14.2 comes the possibility to use a named instance when configuring MSSQL as the external database for the solution.
However, this has some implications which may not be obvious at first sight.
Below we concern ourselves with deploying IDS on a vApp, but the connection information is the same for a more traditional implementation as well.
The first point is to make sure you use the port the instance is running on on the MSSQL server when you configure the external database connections - this is not the same port as the default instance (which usually runs on 1433, but of course doesn't have to). If this port has not been provided you can use SQL Server Configuration Manager to find it out by noting the Process ID for the named SQL instance and then use netstat to find the port, e.g. netstat -abo|find "nnnn" where nnnn is the Process ID.
If you are going to implement Jaspersoft reporting then you should also be aware that using a Named Instance in Microsoft SQL Server for the CABI JasperReports Repository database is not supported in version 6.3. So if your external database is an MSSQL named instance you will need to use the bundled postgresql database for storing the report templates used by Jaspersoft instead of an external one.