Representation clause is just language defined way of how struct (Record in Ada) Enum or Array are actually laid out in memory. So, for example, you can define a struct that represents a register, overlay that register address and use it like this:

  procedure Enable_USB_Clock is
  begin
     Registers.PMC_Periph.PMC_SCER.USBCLK := 1;
  end Enable_USB_Clock;

Example taken from here: https://learn.adacore.com/courses/Ada_For_The_Embedded_C_Dev...