Well I added an individual and this reference ID field did not get populated. When you hover over the field it wants you to link to a source and leads me to conclude that this is not intended to be an individual ID field. Also see this quote from an online blog:
In the
Media section when you RT click on a photo and LINK to person the Index comes up – you then find the individuals name – — before you hit OK – over on the RT hand box – my suggestion is that there be another
TITLE added to the list of information and that is the SPOUSE and the REFERENCE ID #. [I use my own numbering system]