A deep-carry clip is shaped so the knife sits lower in the pocket, leaving only a small amount of handle visible above the seam. It is chosen for discretion and for security, and it is one of the most common aftermarket upgrades.
A standard clip leaves an inch or more of handle proud of the pocket. A deep-carry clip drops that to a fraction of an inch, so the knife is far less obvious in an office or a public setting.
Sitting deeper also means less of the knife is available to catch on a seat belt, a car door or a bag strap, so the knife is less likely to be pulled out accidentally.
Less exposed handle means less to grab. Drawing a deep-carry knife takes a slightly more deliberate pinch, which some people find slower and others do not notice.
Fit is model specific. A deep-carry clip has to match the knife exactly at the mounting holes and clear the lock bar if there is one, so buy for your model rather than by eye.