To me, an offline is a simple get-together. It may or may not have a theme. It may or may not have an industry guest (though "not" is more likely). It's just a handful of people getting together to drink Port.
A tasting is something bigger and more official. A full Vesuvio vertical is a tasting. A full 2007 horizontal is a tasting. A Taylor/Fonseca/Dow/Graham matrix of '63/'66/'70/'77 is a tasting. Needing to break the flights into pre- and post-dinner sessions is a tasting.
I can't explain the grey area in between, but I can tell you whether an event is an offline or a tasting.
