The CRM Lead Source field is typically used to identify which web form a contact submitted or to simply identify the source of your leads.
The system CRM Lead Source field is populated when you supply a value in the Optional Lead Source for Site under the Site Settings in Landing Pages. Some Marketers use this field to identify the source of their leads.
In addition to the Optional Lead Source for Site option, Google Analytics can also populate the CRM Lead Source field in Acoustic Campaign.
In Web Tracking, Lead Sources provide a way to capture specific campaign-tracking identifiers by associating online marketing activity to new contacts when they opt-in to your database from an advertised site, URL or Referrer, after submitting a web form. Lead Sources, is the the primary reference to which a placed banner or ad is a part of. In many cases this will be a combination of placement channel and campaign name.