CC2540 MINIDK: How does the Write or Read CBs work?

I am new to the MINIKD CC2540, I am confused on the CBs.

There are simpleProfile_ReadAttrCB, simpleProfile_WriteAttrCB in the simpleGATTprofile.c file. And they are registered

:GATTServApp_RegisterService( simpleProfileAttrTbl,
                                          GATT_NUM_ATTRS( simpleProfileAttrTbl ),
                                          &simpleProfileCBs );

It says in the Guide: Every time a GATT client devie wants to read from an attribute in the profile, this function gets called.

My question is: When I send "Read using Characteristic UUID" from the bTool (USB Dongle) to the Keyfob, how does the Kebfob response to the read command? What is the detailed routine?

Thank you pro.