Before answering this question let us know what are net and total areas.


Now let us look at first part of our question. Net area of our region will be equal to total area of region when the whole region lies above x axis because then our whole region is positive.
The net area of a region will differ from the total area of the region when some part of our region is above x-axis and some part of region is below x-axis. This is because we have to subtract area of region which is below x-axis from area above x-axis to get net area.