TMS320F2800137: X1CNT counting question

Part Number: TMS320F2800137

Hi,
customer has question to our example code in sysctl.c:

do
    {
        //
        // Keep clearing the counter until it is no longer saturated
        //
        while(HWREG(CLKCFG_BASE + SYSCTL_O_X1CNT) > 0x1FFU)
        {
            HWREG(CLKCFG_BASE + SYSCTL_O_X1CNT) |= SYSCTL_X1CNT_CLR;
        }

        //
        // Wait for the X1 clock to saturate
        //
        while(HWREGH(CLKCFG_BASE + SYSCTL_O_X1CNT) != SYSCTL_X1CNT_X1CNT_M)
        {
            //
            // If your application is stuck in this loop, please check if the
            // input clock source is valid.
            //
            localCounter++;
            if(localCounter>2500000)
            {
                if(loopCount == 3U)
                    status = false;
                break;
            }
        }

        if(loopCount == 3U &&
           (HWREGH(CLKCFG_BASE + SYSCTL_O_X1CNT) == SYSCTL_X1CNT_X1CNT_M))
        {
            status = true;
        }
        //
        // Increment the counter
        //
        loopCount++;
        localCounter = 0U;
    }while(loopCount < 4U);

Why was the value 0x1FFU be chosen for clearing the counter? Any special reason or can another value be used?

Regards, Holger