why to use squared energy and mean amplitude as features of gabor filters?

I was reading a paper about using gabor filters for feature extraction, and the author mentioned that he represented the gabor filter responses in terms of amplitude and energy, so my questions are:

1) what is the meaning of energy in this case? 2) when to use energy and amplitude as features in texture analysis?

3) is there a difference between gabor wavelet and gabor filter?

the paper i was reading : "Local isotropic phase symmetry measure for detection of beta cells and lymphocytes"

Thank you so much