How RBL distinguish UBL and application code

HI,

I am using OMAPL138 for my ARM-DSP application in which SPI1 BOOTMODE is used by ROM BOOTLOADE, and i am using a logic PDexperimenter kit. Also, am using AISGen for creating a BIN file and its written to FLASH using serial flasher tool.

I tried flashing without using UBL , and my Application loaded successfully. from that i understood that RBL itself can load my application (is it correct ..?).

but my requirement is i need a UBL which should load the App.

my question is how the RBL distinguish the UBL from the application...??

Thank you 

Tinu