Moneil:
You really shouldn’t be asking this kind of question here, it boarders on a, first day, classroom question and they are verboten on this site. Everyone must learn someplace or start their education on a subject someplace, but this isn’t a classroom or a textbook on a very important subject. And, your’s is a very basic question for someone claiming to be working on WPS’s. Furthermore, a bunch of acronyms won’t make you smarter if you don’t know what they mean, or the people you are communicating with don’t know what they mean.
You should be taking these types of questions to your boss or supervisor, so he/she knows what you don’t know and can offer arms length, eye contact, with a smile, instruction and advice to keep you out of trouble, which keeps them out of trouble. You must get comfortable with asking your boss questions like this, without embarrassment, to make that mentoring relationship work properly. Also, get some good books on basic welding, the Lincoln Arc Welding Foundation has some very good ones; AWS and AISC too, and study them. That learin will stick with you a lot longer than our answers to your question here. Read connectegr’s answers very carefully, because he did not say that the fillet weld does not fuse to the parent metal of the joint. And, what he might have said is that “CJP welds do fully fuse the cross section......” A small point, but one that you should know, if you are designing welds or working on WPS’s.