Canon EOS Rebel XS Instruction Manual page 55

Advanced Shooting
Techniques
In the Basic Zone modes, to prevent spoiled shots,
most functions are set automatically and cannot be
changed. In the <P> (Program AE) mode, you can set
various functions and be more creative.
In the <P> (Program AE) mode, the camera sets the
shutter speed and aperture automatically to obtain the
standard exposure.
The difference between the Basic Zone modes and <P> is
explained on page 170.
* <P> stands for Program
* AE stands for Auto Exposure