According to this:
http://www.dts.com/DownloadDocument....2-cda9554bb6a5
DTS
Core is the "legacy" audio provided for systems that do not support the
Extension. In older systems, you just listen to the Core. In modern systems, the decoder takes the core
(lossy), then compares the data with the Extension (
lossless) and applies the extra data that needs to be played and are lost in the core to deliver the best possible audio experience.
Bottom line, if you don't have a decoder that support DTS-HD (and I mean a hardware one), just using the Core option to demux and then copy stream, you will hear the
exact same sound you would hear by demuxing the DTS-HD channel and mux it in the mkv yourself. Minus about 1GB of audio data.
So unless you have a home theater equipment (decoder) that supports it, just copy the Core.