PTHREAD_MUTEX_INITIALIZER being typeless

[Yadrif]

I found a comment in code stating that PTHREAD_MUTEX_INITIALIZER is typeless then the following code:

const pthread_mutex_t pthread_mutex_initer = PTHREAD_MUTEX_INITIALIZER;

then pthread_mutex_initer is used to initialize a pthread_mutex_t member variable.

What is meant by is typeless in this context? And why do they need the above code?

Thanks for the help.