I need to create a cross-site parameter that is available to make a field conditional.  This is decided at a site level and it is preferred to allow the site manager to set the parameter in the front end.  The parameter indicates if the the site is two-dimensional or three dimensional.  Using conditional settings on field 3, if two dimensions, fields 1 and 2 are visible and field 3 is hidden.  If three dimensional, then all three fields are visible.  If I use a simple select field, the parameter has to be set for each article created using the content type.  If set in the back end as a default in the content type, it means that all articles already created do not pick up the parameter dynamically.  I have also tried using Multisite but the field created is not available to set up the conditional in field 3.

Advice gratefully received