Ambrose LI wrote:

Also, we may need to combine it our other codes, can we modify
the codes for customization?

Yes, Koha is licensed under the GPL; you certainly can modify
the code for customization.

If you use your customization internally, there is no problem, absolutely no one.
If you plan to sell your customization, or give it to any other library, there is only 1 "limitation" : you MUST place your modifications under GPL, and allow customer to modify/redistribute your customization.

-- 
Paul POULAIN
Consultant indépendant en logiciels libres
responsable francophone de koha (SIGB libre http://www.koha-fr.org)