Hi, thanks; isn't it strange to have it there and not in the Transport
tab in Preferences, where all the other sync options are?
the choice of external clock is considered to be session specific, not global to ardour. in some sessions you might sync to LTC, on others to MTC, on others to JACK and on others just use the internal clock. thus, it is not a global preference and so (like all other session properties) it lives in Session > Properties, not in Edit > Preferences.