Trigger & Freewheel......

Or Full Chase Lock?

Which one and why?

Default is Full Chase.

I record mostly audio with a dash of MIDI.
Is one setting better for audio and one for MIDI?

I do have a word clock jack on my 18i6 but I don't have an external clock.
Does this mean there's a clock in the interface or just a way to add one if needed? 

Mike

AFAIK this only applies to external (e.g. SMPTE for film and TV) synchronization. It determines how SONAR tries to keep up with a film editor or hardware sequencer. Trigger & freewheel means SONAR starts up when the external device tells it to and then rolls along at its own pace from there on. Full Chase Lock means it's continuously syncing up via the SMPTE timecode, which I presume allows for tighter alignment between devices. 

Pretty obscure functionality for the vast majority of SONAR users. Bottom line is you probably don't care one way or the other.