I've mentioned it already.
A non modular unit will have all the cables some pre-assembled with the unit. After you've built your system, you'll quickly realize that the leftover cables are a real hassle to deal with. Depending on the case you'll choose this ordeal will either be greatly simplified or become a horror story worth pinching yourself out of. Simplified if your case has room behind mobo tray to hide them cables.
A semi modular unit has
some cables pre-assembled with the unit. These cables are usually the 24pin mobo connector and the 12v EPS cable. The rest are added upon the users needs/requirements. Similar to the previous pointer the cable managing becomes easier since you have fewer cables to deal with.
* This thread has been posted on the GPU section, if you're going to start with other questions then we either need to start a new thread or literally reroute the thread location.
So what's it gonna be ?