If you are downloading new SCCM/ConfigMgr CB (Current Branch) source files then you should download the latest “Baseline” version of SCCM ConfigMgr CB 1606. You can download the source files of SCCM CB from 1. Eval center here (for free – 180 days trail version) 2. MSDN here (personal lab testing when you have MSDN subscription etc..) and 3. Volume Licensing site here (arguably, only for production, pre prod and lab for official purpose). The title which you should look or search for is “System Center Configuration Manager and Endpoint Protection (version – 1606) (x86 and x64) – DVD (Multiple Languages)“.
Until now we were downloading the baseline version of SCCM CB 1511 from MSDN or Volume Licensing site and upgrading the version of SCCM CB via “Updates and Servicing“. What is changed now? Recently, Microsoft of released new baseline version of SCCM ConfigMgr CB and that baseline version is SCCM/ConfigMgr CB 1606. That means you don’t have to download and install SCCM CB 1511 and upgrade it via Updates and Servicing node to 1606. Rather you directly download SCCM CB 1606 version and install it. While installing the new baseline version of SCCM CB 1606, you could see an option to select “Long Term Servicing Branch (LTSB)” never ever select that option (why ? – more details here).