# MSP430 UCS - FLL/DCO Source

I'm trying to understand better the whole clock system mechanism in TI's MSP430.

I understand that when doing this line of code I'm assigning REFO as the clock reference to the FLL/DCO

UCS_clockSignalInit(
UCS_FLLREF,
UCS_REFOCLK_SELECT,
UCS_CLOCK_DIVIDER_1);


However, according to documentation I can pass to UCS_clockSignalInit other values among those is UCS_DCOCLK_SELECT and do the following:

UCS_clockSignalInit(
UCS_FLLREF,
UCS_DCOCLK_SELECT,
UCS_CLOCK_DIVIDER_1);


This is unclear to me. From what I understand the function call sets the DCO as the reference clock to the FLL/DCO itself. Can someone explain to me how/why is this possible.