As we know, lists contain various columns within a site. These columns cannot be shared cross sites. For this SharePoint provides an ability to create columns which could be shared across sites (Site Collection) providing reuse of columns and possible elimination of duplication of column creation.
Basically lists inherit site columns. Which means that, when a site column is modified, changes can be pushed down to the list except in scenarios where in site column itself removed (due to possible usage of the column in the list already). For example, SharePoint already has some site columns defined like Start Date, First Name, etc.
Site columns can be created using Site Settings of Top level Site. Once created, it is immediately available to all the lists down under. If any changes are made to Site Columns, changes can be optionally pushed down to the lists which already use them. Otherwise, only the lists which are created henceforth will have new changes.
Using Site Column in a list
- Navigate to the list in which you need to use site column.
- Select Actions -> List Settings
- Select Add from Existing Site Columns link under Columns section.
Move the required columns from left pane to the right and save it.