Difference between "Table-Incremental" and "Table-Update" for "Population Type" when scheduling Full Text Index Population
By default the Full-Text Engine uses automatic population on a full-text index and changes are tracked as data is modified in the base table, and the tracked changes are propagated automatically. The full-text index is updated in the background. 

For incremental population your table must have a column of the timestamp data type. At the end of a population, the Full-Text Engine records the largest timestamp value and will use that for next incremental population

Table-Update will use change tracking to maintain a full-text index after its initial full population. The SQL Server will then maintain a record of the rows in the base table or indexed view that have been modified by updates, deletes, or insert and will use this information to populate the Full Text Index. This comes with an overhead. 

In most cases you are better off using Automatic updates, but if you have a table that's frequently updated and you have a timestamp column a manual Table-Incremental can help. (https://technet.microsoft.com/en-us/library/ms142575%28v=sql.100%29.aspx)