I never really understand the various sync options and just use the "Trigger and Freewheel".
Now I have intermittent issue with virtual drum (BFD) not in sync with the  project's timing and try see which is the best sync option.
My sound device is a Lynx Hilo USB, and my question is:
What exactly is ""Trigger and Freewheel" and "Full Chase Lock" and when to set the various setting on the Sync page?  Also Sonar shows that the Hilo as midi device, do I select that too? thx
 

That's for syncing to an external sequencer or drum machine, nothing really to do with what's in the box. 

If you're not looking for deep explanations, you might find some answers here.
 

Thanks for the info.  So how do Sonar sync with virtual Drums, through vst protocol? and how does it sync with the sound card, in my case, would that be the USB driver?

With a computer-based sample player such as BFD, there should be no sync issues, and the settings on your audio interface should be irrelevant. That's because timing is handled entirely within SONAR. I have never seen a soft synth that wasn't right on the money (I'm assuming BFD is being driven by a MIDI track, rather than being played on an external kit). Issues are far more likely when driving external synths.
 
So why is BFD exhibiting timing problems? I suppose there could be some unaccounted latency in some plugin. Test that by removing everything from the project except the drum track and see if the problem goes away. Sometimes, plugins with very large latencies (e.g. Ozone with large buffers) can require increasing the "prepare using N millisecond buffers" setting.

A VST instrument like BFD is driven by the midi track. It should play each note as the event happens. 
I believe Lynx audio interfaces are solid good performers. They also have low round trip latency. 
But as Dave said, often what we call midi latency can be caused by certain plug ins that use look ahead buffers etc. 
So if what you saying is you hit your keyboard but the note takes a second or more to play, then by pass your effects bins and see if that solves it. 
 
And the Lynx if it has midi ports will show in Sonar as an available midi device. 
If your not using the midi ports then they have nothing to do with your issue. 
 

You guys are great.  The issue appears to be fixed by increasing the Midi setting "prepare using N millisecond buffers" from the default 250 to 500.  Interesting that Sonar recommends 500 or higher but defaulted to 250.
I've struggled with this issue for a while but it's manageable.  When I try to use the "External Insert" to route the mix to Hardware, it basically became unusable, as you have pointed out, due to large round-trip latency.
Again a million thanks
 

One caveat: if you're using the 32-bit BFD Eco. Bitbridge has a bug that will add an empty buffer of audio to the front end of rendered output, making it play back audibly late when the audio buffer is high.
 
Having the MIDI Prepare Using buffer too low doesn't generally cause timing errors; the more common symptom is dropped notes. I suppose that might sound like a timing error in some cases, but otherwise, I'd be surprised if changing the MIDI buffer really solved a sync problem.
 
External Insert latency depends on your round-trip audio latency, and the latency of whatever hardware you're running the signal through, which can add up if the hardware uses DSP with analog I/O and has to do its own A/D/A conversion on top of SONAR's. And SONAR's automatic Plugin Delay Compensation is going to delay everything else to maintain sync with the EI return, causing the whole project to have more latency. External Insert processing is usually something you wouldn't use until all tracking has been done, and you've moved on to the mixing stage where playback latency doesn't much matter.