What's the exact meaning of bond population in the output file of castep?

Hi,everyone:
I did some calculations about metallic oxide absorption on crystal to study the effects of metallic oxide absorption on properties of crystal wiht CASTEP. I found that bond length of the absobed moleculars became short while the rest became long. I wanted to analyze this change by bond population. So what's the exact meaning of bond populations in castep ? And how to calculate the bond populations?