I have set up a list of valid entries for a cell using the datavalidation
tool in Excel. Lets call this the primary list. I now want to set up another
list of entries, but I want the choice/s in this secondary list of entries to
be restricted/determined but the individuals' selection from the primary
list. How do I go about creating the restrictions in this secondary list?

To add more meat to the bones. The primary list is a list of four companies.
The secondary list is a list of sites where the four companies operate. No
two companies operate on the same site and three out of the four companies
operate on more than one site.

Any help would be much appreciated.

Thanks

Gruntos