Kaise hai yeh Dual Taxation?

Maine pehle hi samjha tha ki section 90 mein dual taxation ka concept hai, but abhi tak maine nai samajhya ki yeh concept dono countries ka kya relationa hai or kaise work karta hai. Maine suna hai ki dual taxation ka matalab hai dono countries mein income tax ka payment. Lekin agar ek vyapari ka income dono countries mein aata hai, to uska 90 ka fayda kaun lene wala hai or kaise lene wala hai? Kya dono countries ki rates ke hisaab se 90 ka fayda hota hai ya phir ek hi tarike se hota hai.