can someone please explain the difference
thanks

The crusting buttercream has a slight crust as the icing sets, hopefully someone else can answer the non crusting. Hope this makes sense and helps

The crusting buttercream (BC) is for smoothing. Non-crusting BC doesn't dry~ I think I'm right on that... someone will correct me if I'm not...

Yepper that is pretty accurate. I guess I break it down a bit further. Your typical class buttercream with the all-shortening and water as a liquid and meringue powder - that is a crusting buttercream and like my Turtle friend said, it crusts on the outside. So your flowers will crust up a bit or air dry, the outside will be a bit drier.
A recipe for buttercream that uses butter and shortening and milk or cream will set, not really crust.
Both of these buttercreams will set up or dry in about 25 minutes so you can then try any of the popular methods of smoothing your icing, like with a paper towel or hot spatula and such.
Then there is also an air-drying buttercream, which is almost the same as the class buttercream but has a bit more of a set-up or crusting effect and this is good for times when you want to make a cascade of roses or such, on the sides of your cake.
A nun-crusting buttercream, well it doesn't set or crust. So you cannot really use many of the methods we use for smoothing the icing out. Nor will the roses and such set up quite as well. It will always be somewhat more wet to the touch, so if after 25 minutes or so, when you touch it with a paper towel, the icing will come off onto the paper towel. Similar to how whipped cream would react or canned icing.
Hugs Squirrelly