I think discourse is the way how a certain essay is written. Discourse could relate to formal or informal ways of writing. PTE looks for more formal writing since it is considered academic type of writing.
So the reason somebody could score low in discourse but high in writing as a whole could be that he/she wrote an essay that is strong content wise but maybe was written in a slightly informal way.
Words like “i think” , “we can” are informal ways of written discourses and would score low on the PTE written discourse section whereas “it would be wise to say” and other formal written forms would score high in the written discourse section. An essay with strong content would undoubtedly score high in written overall but the score on the written discourses would vary depeding upon your type of writing, formal or informal.
All in all, if you write formal english, you would score high ln written discourse and if you write informal you would score low. Written Discourse in its individual terms means the manner an essay is written.
These are all predictions though. Even i have no idea of telling what it exactly is.
Hope that helped.
sudeepdai (PTE – Academic Score: R-90 L-90 S-90 W-90 Overall – 90)