CrPC vs BSNR: When Does Investigation Begin?

Yaar samajh nahi aaya, koi bataaega? In CUET PG Law ke exams mein, mere paas do concepts aaye the CrPC aur BSNR ke liye, joh bahut hi similar lagte hain. Mere saamne yeh sawal aaya hai kyun ki investigation kab shuru hoti hai - CrPC ke article 156 ke liye, ya BSNR ke section 157 ke liye? BSNR ke section 157 ke according to, investigation shuru hoti hai jab FIR registered ki jaati hai ya police ko complaint milta hai. Lekin CrPC ke article 156 ke according to, investigation shuru hoti hai jab police ko duty assign ki jaati hai. Kya yeh dono concepts ek dusre se different hain? Ya koi confusion hai?