Who can explain RSA key types for me?

[Zuxy]

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When running "gpg --expert --gen-keys", I have 6 key types for
selection:

(4) RSA (sign only)
This is somewhat a DSA equivalent which can be used with a subkey for
encryption?

(6) RSA (sign & encrypt)
This is a v4 equivalent for v3 RSA keys? One key to do it all?

(8) RSA (sign & certify)
What's this? And what's the meaning of "to certify"? Does gpg use
"sign" for signing data and "certify" for signing other's pubkeys? 

(10) RSA (sign, encrypt and certify)
And this one?

And, when running "gpg --edit-key xxxxx", I note something called
"key usage". Again, there's "C" for certification, "S" for signing,
"E" for encryption and "A" for authentification. So what's the
essence of these four letters (especially the difference between C, S
and A)?

Thank you all!
 
- -- 
Zuxy
Beauty is truth,
While truth is beauty.
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