求助 结构体定义

#define MAX_SIZE 200
//***************************************
// 结构体定义
//***************************************
struct Dat2
{//报警信息或报警相关信息数据结构
    unsigned char           
    FsqAddr,Comm,Main_addr,Loop_addr,Device_Type,Device_H,Device_L,
        /*楼*/Floor,/*层*/Layer,/*区*/Qu,Year,Month,Day,Hour,Min,Sec,
        ABC1,ABC2,ABC3,ABC4,ABC5,ABC6,ABC7,ABC8,ABC9,ABC10,
        ABC11,ABC12,ABC13,ABC14,ABC15,ABC16,ABC17,ABC18,ABC19,ABC20,FSQ_CRC;
    struct Dat2 xdata *next;
    struct Dat2 xdata *prior;
};
struct Dat2            
    xdata    *outMsg,    xdata    *outMsg_Mrr,
    xdata    *frHead,    xdata    *frTail;         //fire


typedef struct
{
    struct Dat2  elem;
    char used;//0:unused, 1:used
}Dat2Map_t;
xdata Dat2Map_t Dat2Map[MAX_SIZE];
这么定义有什么意思？