TMS320F280049: Question about SCI driverlib function

Hi team,

Here is a question about SCI example in driverlib of F28004x.
In the follow line:

while(SCI_getTxFIFOStatus(base) == SCI_FIFO_TX15)

Why the condition is FIFO level == 15 rather than 16 which is full status?

Thanks!

void
SCI_writeCharArray(uint32_t base, const uint16_t * const array,
                   uint16_t length)
{
    //
    // Check the arguments.
    //
    ASSERT(SCI_isBaseValid(base));

    uint16_t i;
    //
    // Check if FIFO enhancement is enabled.
    //
    if(SCI_isFIFOEnabled(base))
    {
        //
        // FIFO is enabled.
        // For loop to write (Blocking) 'length' number of characters
        //
        for(i = 0U; i < length; i++)
        {
            //
            // Wait until space is available in the transmit FIFO.
            //
            while(SCI_getTxFIFOStatus(base) == SCI_FIFO_TX15)
            {
            }

            //
            // Send a char.
            //
            HWREGH(base + SCI_O_TXBUF) = array[i];
        }
    }