z-stack SampleApp endpoint

HI,

I  met  a  problem  here.

there  is  a  definition in  the z-stack. Just  take  the  following  description  as the example.I  can   not  understand  what  the endPoint  means  here.

Would  you  help me?  I  would  like  to find  the answer .  Thanks  for  your help....

/**************problem begins******************/

typedef struct
{
  byte endPoint;
  byte *task_id;  // Pointer to location of the Application task ID.
  SimpleDescriptionFormat_t *simpleDesc;
  afNetworkLatencyReq_t latencyReq;
} endPointDesc_t;

        // "#define SAMPLEAPP_ENDPOINT           20"              // The  definition  "20  "     represents  what ???

       //SampleApp_epDesc.endPoint = SAMPLEAPP_ENDPOINT;

       //endPointDesc_t   SampleApp_epDesc;

/******************problem ends***************/

Br.

Qian