Sharepoint Site Columns are fields that can be re-used across sites and sub-sites while creating lists or document libraries.
SharePoint provides extensive list of build-in site columns that can be used by the end-users to create and manage their content. We’ve categories like: Base Columns, Core Contact & Calendar and each of these categories contain site columns like Start Date/End Date, Address, City, Phone etc.
Creating Site columns
1. Open your SharePoint 2013 site.
2. Click on settings icon and select Site Settings.
3. From Web Designer Galleries select Site columns.
4. Click on Create to create a new site column.
5. Provide a Column name and Column type
6. Select existing group or create new group
Adding Site Column to your document library
1. Select a document library and open Library Settings
2. In Library settings select Add from existing site columns.
3. Select Group to view site columns of that group
4. Click on Add to select column then click on Ok to add that column to your document library
5. Document Library has the column which we created and added to the default view