All the points you selected will be at the exact elevation you requested.
However, the elevation points just outside your selection do not change.
So, if you raised, or lowered your selected area to say, 10m, the area surrounding your selection remains unchanged.
You can of course select all the points surrounding your selected area afterwards and use the Up/Down function to raise or lower them all by a certain amount.
This will keep the basic terrain shape it had before, it just lowers it a little, or more, depending on what you input and how many times you apply it.
Planck