Where a block was divided by a tract line, the block was divided proportionally to how the block was divided. A block with 10 people divided in half would assign five people to one tract, five to the other. This method estimates what 2000 tracts would have held in 1990. The Census Bureau also provides data for what they call Census Designated Places. Those can be cities, like St. Petersburg or Brooksville, and they can be unincorporated places like Westchase or Holiday.
Places can change from census to census.
Some new places are added, old places are dropped, and lines are expanded. Places that were counted in the 2000 Census for the first time will include zeroes for 1990 numbers, and therefore would not include any percent change calculations. That does not mean, however, there were no people living within those boundaries 10 years ago. Areas that are not designated as places are left unbounded and show up in white on the map. For information on those area, see the tract maps.