One of the many new features in Office SharePoint Server 2007 is the availability of sitecolumns. Sitecolumns are SharePoint list fields, that are defined at the site level. They are a general definition of a column, that can be re-used throughout the site, and it’s subsites. This is an important improvement, because you probably all remembered adding a choicefield “Region” to your lists/libraries in SPS2003. Just after you finished adding this choicefield to all lists, you found out there was a new region that had to be added to all the dropdowns…..
This is where sitecolumns in MOSS help you. Another great thing is that you can now create a “Customer” lookup column, and re-use that throughout all your sites. This way all “Customer” lookups use the same list that holds the customer data. Nice!! These lookups also have a nice new feature: “Allow multiple values”. 

The following is a brief description of how to create site columns. The first step is to create the site column. In any site, go to Site Settings in any site and click “Site columns” in the Galleries section, and click “Create”.

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An important thing to remember is that site columns have a “scope”. The availability of your sitecolumns in the portal/sitecollection depends on this scope. Site columns (this also applies to content types) are available in the site where you create them, and all it’s subsites. Site columns that should be available in every site, should be created at the root level (Home). After you create the site column, you can use that column in all your lists and document libraries (or even better, in content types).

The next step is to add the site columns to a list or document library. Go to the settings menu of your list and click the “Add from existing site columns” link:

When you make an update to the select list, the changes are automatically pushed to all instances, when you select “Update all list columns based on this site column?”  

For each instance of a site column in a list, you can override settings (like “required” or the default value). Please note that when you make changes to the site columns itself, and update all the lists, these changes at the list level will be overridden.