The Campaign Field History related list of a campaign detail page Track changes to standard and custom fields on campaigns. Every time, anyone user makes changes to any of the standard or custom fields, history is set to be tracked on the campaign, a new entry will be added to the Campaign Field History related list. All entries will have details of the date, time, nature of the change, and who made the change. Changes made to the related lists on the campaign will not be tracked in the campaign history.
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Also, watch this video on Campaigns in Salesforce: