The Last Source field reflects the source value most recently encountered for a particular record. Unlike Source, which should never change over the lifetime of a prospect, Last Source will change frequently to reflect the method that most recently updated the record in eTrigue. Update methods include list imports, form fills, or manual prospect record updates.
As an example, John enters the database for the first time through filling out a "Contact Us" form -- the result is that his Source and his Last Source are both set as "Contact Us" form. A month later, John is included in a list import that attempts to assign the Source as "Tradeshow List XYZ" -- the result is that John's Source remains as "Contact Us" Form, but his Last Source value changes to "Tradeshow List XYZ" to match the most recent Source with which he was associated.
Since Last Source will change frequently, it is not recommended that this field be used for criteria in lists. Prospect categories are recommended when you want to assign people to a single searchable cohesive group.