hardcoe Mode adds:
- Ammo weight instead of weighless ammo
- Stimpack healing over time instead of instant stim healing and limb repair through doctors or doctor's bags + skill check
- The need to eat, drink and sleep regularly or suffer negative status effects and if you go without a combo of the 3 long enough, even death
You have the option to turn it on in the beginning and you can turn it of anywhere in the middle if it proves too challenging, but you lose the chance to win the hardcoe achievement/trophy if you do so and you cannot switch it back on.
You can also change the difficulty slider any time so if you want a challenge go hardcoe mode and choose the very hard difficulty.
Personally I'm going with hardcoe/normal and I reserve the right to slide the bar towards very easy in the beginning if supplies prove to be too scarce and towards very hard towards then end when I've built up a personal mini arsenal and supply store...