A basic question about File System Library

Where can I find guidance about how to use the function in FS Library? I'm trying to use these functions in rl_fs_lib.h (from MDK-Middleware 7.11.1) but don't know how to pass the pointer properly

and also I'm trying to partition a TF card into several volumes and drive them seperately by change the DrvLet string in the code below:

Does someone have insight about what function can be used to assign values to this DrvLet?

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