The Selection tool is the 5th icon in the side-bar.
When an area is selected, you can deselect by tapping on the same tool.
The additional side-bar displays options relative to the selection.
This option allows you to switch automatically to the transformation tool after selecting an area.
Enables the zoom and pan gestures. If it isn’t enabled, you will be able to transform the image with your fingers. When enabled, the transformation can only be made by handling the transformation box and you can zoom and pan with your fingers.
The Free-Hand selection allows you to draw the shape of your selection.
The Rectangle selection allows you to draw a rectangle of the area you want to select.
The Ellipse selection allows you to draw an ellipse of the area you want to select.
Makes a selection of all the pixels contained in the current drawing sheet.
Inverts the selection.
Once you have set your selection, you can switch to the Brush tool to draw or fill only the selected area, or switch to the Transformation Tool to move and/or resize the selected pixels.