Now keil says
In-System Programming means that the device can be programmed in the circuit by using an utility such as the ULINK USB-JTAG Adapter.
In-Application Programming means that the application itself can re-program the on-chip Flash ROM.
Also UM10524 says
In-System programming (ISP) is programming or reprogramming the on-chip flash memory, using the bootloader software and the UART serial port. This can be done when the part resides in the end-user board.
Does that mean ULINK interfaced with SDDIO & SWCLK pins, access bootloader software & UART0 of the LPC then program the IC.
However I debug my code using USART0 with ULINK. It worked fine. No error. As by above statements ULINK will access UART) to program however I am using USARt0 to send code on PC terminal.
Or I misunderstood it.
"Does that mean ULINK interfaced with SDDIO & SWCLK pins, access bootloader software & UART0"
No.
The uLink uses the on-chip debug hardware to write directly to flash; it does not use a bootloader.
In-Application Programming (IAP) means that your running application updates its own flash (or, more commonly, your application delegates this task to a so-called "Bootloader").
"I debug my code using USART0 with ULINK"
I rather doubt that? Again, the uLink uses the on-chip debug hardware - not the application UART.
"I am using USARt0 to send code on PC terminal"
That will be using a bootloader.