What is GnuPG UID for?

[Fernando Shayani <fernando_shayani@yahoo.com.br>]

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Can anyone help me understand what is the GnuPG User ID is for?

I understand that I can make more than one UID with the same keypair,
but what's that for? And how can I choose witch one I want to use to
sign the message each time?

I'm using WinXP with WinPT.

Thanks!
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