I've seen some of these settings explained over at Operation Sports.
Some are self explanatory but really, when do you use "Switch Everything"?
What is "Four Out Corners Tight" or "Boxes & Elbows".
Anyone, please feel free to pitch-in.
Let's help the community play some damn good defense.

1. Auto - I don't use ACE. I manually adjust "Defensive Settings"(Moderate, Deny Ball, Hard Hedge, etc.) Do these on-the-fly settings override your manual adjustments?
2. Switch Everything - Good to use when opponent keeps pick & rolling ? Will mess up your match-ups.
3. Four Out Corners Tight -
"No, it's more used against dribble drive offense that count on someone to penetrate and collapse the defense. Usually when the team only has 1 great perimiter player The "four corners" is referring to the four corners of the paint. This is basically how it works, your best perimeter defender defends there best perimiter player. Then there's two high box players who usually set up around where the free throw line and paint meet. Then the two low box players... Sit on the two low blocks of the paint. Walling off the paint From that player. Leaving the 3pt line kinda wide open. Which in real life is okay in some situations (starting to get less and less utilized as the game evolves. ) Never use this in 2k unless you're wanting to get shredded due to the overhaul of 3pt shooting." - MyBBalliQisDecent (Reddit)
4. Stay Home - No double team. Defenders will stay with their man no matter what.

5. Straight Up - Moderate "On-Ball", Moderate "Off-Ball".
6. Shrink The Floor - Lock down the paint.
7. No Threes - Tight perimeter defense.
8. Neutral - Cancels any setting you made ?

1. Auto Trans - (New settings for 2K20) I guess this is defense for fast breaks.
2. Protect Perimeter - When opponent loves shooting 3's during fast breaks.
3. Wall Up - When opponent likes to attack the rim during fast breaks.

1. Auto - Why even use auto?
2. Give it Up - Suffocating defense. Smother on-ball, keep doubling, don't give him air to breathe. The only option for him is to pass the ball.
3. Nothing Inside - Good for slashers or dominating big men.
4. Force Jumper - Good for awful shooters.
5. Run Off Line - Deny him the ball? **Unclear

6. Make Him Score - Gap defense. Good for low rating players.
7. Stay Home Under - (New 2K20 setting) No double team, go under when there's a screen ?
8. Stay Home Over - (New 2K20 setting) No double team, go over screen ?
9. Force Drive - Good for non-slashing "pure shooters".
10. Boxes & Elbows - 2 guys on the block(boxes) and 2 guys on the elbows.
"A man to man concept (can look like a zone when ran) where the player it is selected for will have his man guarding him and the other 4 defenders on the two elbows and just outside the paint on either side of the rim. You are giving up shots to get the ball out of the selected player's hands." - NBA 2K17 Defensive Settings Community Wiki (Operation Sports)
"Toronto straight up going Boxes and Elbows defense every time down. Showing multiple guys in every gap. Doing it effectively, only way to truly contain Giannis." - Sam Dekker on TOR vs MIL, 2019 NBA Playoffs(Twitter)

11. Boxes & Elbows Jazz - (New 2K20 setting) What? **Unclear
12. Boxes Post - (New 2K20 setting)**Unclear
13. None - Default
14. Trap the Box - Kind of like a "Drive Help Rules". When player attacks basket, someone will trap him(double)/help defend when he gets to the rim.
15. Jordan Rules - Badder version of "Give it Up"? Should you use this on god-mode superstars?

16. Wall Up - Transition D. Good for "One Man Fast Breaks"
17. Protect Perimeter - Transition D. Good for "Three Point Shooting One Man Fast Breaks"
Junk defense is gone.
Still nobody knew what Grey 5 & Grey 5 Elbow Iso was(NBA 2K19), tut-tut.