There are two types of shared columns :
Select Linked Shared Columns
Use this option to automatically select all columns linked to the currently selected Shared Column.
Select Original Shared Columns
Use this option to automatically select the Shared Column that this column is linked to (reverse of the above "Select Linked Shared Columns" option).
Link to Shared Column
Use this option to Link the selected column to an existing Shared Column. A window will appear providing a list of available Shared Columns.
Make it Shared/Unshared
Make the currently selected column into a shared column. Alternatively, you can unshare the column.
This dialog box will be displayed when a new shared column is being created :