How to use c union nested in struct with no name

I'm working on the so called Hotspot open source project, and looking at the implementation I found a nasty nested union in struct looking like that:

typedef struct RC_model_t_st
{
    union
    {
        struct block_model_t_st *block;
        struct grid_model_t_st *grid;
    };
    /* block model or grid model    */
    int type;
    thermal_config_t *config;
}RC_model_t;

As far as I'm aware in C/C++ that union is unaccesible. So how someone can make use of union declared in such manner and for what purpose?

Thanks!