Well, this is framed work; a completed product, and they specify a depth of 2cm, so I would expect it to be overall outside dimensions. (But don't take my word for it!)

When purchasing just a frame, the dimensions - 16 x 20, etc are inside the rabbet and represent the glass size and size of the mat mounting the artwork (the actual cut internal dimension normally has a little extra allowance).

But I have to say this is an area where online vendors seem to find ways to be as ambiguous as possible!