What is "CommandCore"?
The only reference google finds on your site[1] is "this is a free text field that points to a 'Core' command that is outside of the solution" on [2].
I neither found CommandCore in RWB's XML view nor did I find it in MSDN's RIbbonXML-Reference.
[1] https://www.google.de/search?q="commandcore"+site%3Aribbonworkbench.uservoice.com
[2] https://ribbonworkbench.uservoice.com/knowledgebase/articles/81981-2-customising-standard-ribbons
Hans Müller
shared this idea
Command Core is used to reference a Command by ID when the command is defined in the managed layer rather than the solutions unmanaged layer.
Consider if you customise an out of the box system button. Only the button is added to the unmanaged layer – the command is still managed and so it is no in the solution. For this reason, the Command drop down will be empty on the Button, but the CommandCore property will contain the ID of the command that is referenced.
Hope this helps