How do you use your OpenPGP card ?

[Dany Nativel]

I've been able to generate my primary RSA signing key off-card as well 
as an encryption RSA key.

When I execute the "keytocard" command it asks me where I want to store 
the Primary key :

(1) Signature Key
(3) Authentication Key

I tried both and they work and I wondering what it the best practice out 
there.
What's behind the Authentication Key ?

Dany

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sec  1024R/48D72CFD  created: 2005-01-09  expires: never
ssb  1024R/7E360C43  created: 2005-01-09  expires: never
(1)  NUMBER5

Command> keytocard
Really move the primary key? (y/N) y
gpg: detected reader `GemPC410 0 0'
Signature key ....: 8422 DA92 7A7F 6BAB 608F  D3AF 6E35 E902 3186 D0DD
Encryption key....: A1D1 CE23 AAC1 1135 1742  5C0C A953 9748 7FFB 5067
Authentication key: 7CBB 67EA 4845 9535 4F3A  F188 61A6 A1A2 504D 2B68

Please select where to store the key:
   (1) Signature key
   (3) Authentication key
Your selection?