Select image region
Select
To select image region use two tools
Z Set Slices Range Tool (double slider in 2D view)
X and Y Rectangular Selection Tool (2D view's lower panel) [1]
Fig. 1
Fig. 2
[1] Rectangular Selection Tool is active in default mouse mode.
Fig. 3
Examples
Extract selected region Filters->Extract selection
Erase selected region Filters->Erase selected region
Erase inverse selected region Filters->Erase inv. selected region
Create mask Filters->Mask->Selection to mask