It’s hard to find any good scholarly references on the differences between pro drop and diary drop (I guess because they are uncontroversial and not particularly subtle). However, section 4.2.1 of this MA dissertation has a decent, though very brief, summary: https://awweir.files.wordpress.com/2017/04/weir-ug-diss.pdf Similar observations are also made in the following paper: https://core.ac.uk/download/pdf/76381796.pdf