Why do some libc symbols have WEAK binding and others GLOBAL?

Why are different symbols in libc declared with different bindings, 1510 0x0003d200 0xf7d55200 WEAK FUNC 55 system 454 0x00067b40 0xf7d7fb40 WEAK FUNC 474 puts 147 0x000303d0 0xf7d483d0 GLOBAL FUNC 33 exit From readelf -s /lib/i386-linux-gnu/libc-2.27.so, 1510: 0003d200 55 FUNC WEAK DEFAULT 13 system@@GLIBC_2.0 454: 00067b40 474 FUNC WEAK DEFAULT 13 puts@@GLIBC_2.0 147: 000303d0 33 FUNC GLOBAL DEFAULT 13 exit@@GLIBC_2.0 [From this question](https://reverseengineering.stackexchange.com/q/19764/22669) , it seems like the authors' libc has a LOCAL exit? What the rhyme or reason behind the different libc binding levels?