A. Custom platform BSP, means you can have your-own platform for OpenCL. You can integrate some IO interfaces, or other FPGA IP(s) into system.
B. After .aocx is created , you need to run command 'aocl program /dev/acl0 this.aocx' to configure FPGA , then in host application, it need to get this.aocx content for OpenCL standard procedure to create kernel to run.
C. For your project you may need:
1) Custom you BSP, modify iface\acl_iface_system.qsys & system.qsys, board_spec.xml...
2) Write OpenCL kernel code.
3) Write host application (Linux application)