What do all these difficulty options mean?

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I'm getting ready to start a career game of Kerbal Space Program, but I don't understand what some of these options are. In particular, what is the effect of:
- Allow Quickloading
- Auto-Hire Crewmembers before Flight
- No Entry Purchase Required on Research
- Allow Stock Vessels
Of course, if someone wanted to elucidate all the game difficulty options, and not just these, that would certainly be welcome.
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