If you include the optional Dynamic keyword, the new column becomes a dynamic computed column, meaning that subsequent changes made to the source table are automatically applied to the destination table.
If you create a dynamic column, and then close the source table used to calculate the dynamic column, the column values are frozen (the column is no longer updated dynamically).
Similarly, if a geographic join is used in the creation of a dynamic column, and you close either of the maps used for the geographic join, the column values are frozen.