Cross-style ballpoint ink refills are most often found in 7 mm pen kits, like Slimline, Streamline, Easyline or Round Top European. To install a cross refill, slide it through the transmission and screw it in place.
Parker-style refills are installed into the pen barrel with a spring. They're most commonly found in twist or click-style pen kits, like Maple Leaf, Cigar, Sierra, Luxor and more. Parker-style gel refills give the writing feel of a rollerball pen. Schmidt Easyflow 9000 refills are an upgraded rollerball-style ink that fit pen kits that take Parker-style refills.