Taken from the AngularJS 1 documentation:

You can also make the binding optional by adding ? : <? or <?attr.

How does the optional one differ from the non-optional one for the one-way binding?

I can seem to figure out the differences for the optional version of two-way (=) and delegate (&) bindings here on my fiddle: /, but not for the one-way one.

By the way, a very Merry Christmas!