(By Jeremy Collins and Jay Latarche): "An article is a marker that accompanies the noun and expresses notions such as (non-)specificity and (in)definiteness. Sometimes these notions of specificity and definiteness are summed up in the term 'identifiability'. The formal expression is irrelevant; articles can be free, bound, or marked by suprasegmental markers such as tone. Articles are different from demonstratives in that demonstratives occur in a paradigm of markers that have a clear spatial deictic function. As demonstratives can grammaticalize into definite or specific articles, they form a natural continuum, making it hard to define discrete categories, but to qualify as an article a marker should be used in some cases to express definiteness without also expressing a spatial deictic meaning."
In the Grammaticon, articles are bound forms (in Grambank, "free" is intended to be defined phonologically), and they are always forms, not "suprasegmental markers such as tone" (but the latter are quite rare).