Kya hai yeh "Best Evidence Rule"?

Main toh socha tha ki yeh rule kisi bhi situation mein best proof ko consider karta hai, lekin maine ek example mein dekha hai jismein, police ko aapne crime scene se kuch evidence recover hua hai, aur do witnesses bhi hain jo incident ko sawaakte hain, lekin unka testimony alag-alag hai. Toh police woh best evidence use karta hai jo usse suitable lagta hai. Lekin yeh to confusion hai ki yeh best evidence to kaisa define hota hai? Kya yeh best evidence ki aapne witness kee statement ho, ya fir physical evidence ho? Jab tum log mock tests dete ho, I'm at my shift, main yeh solve nahi kar pata.