user61079: This website ask me to enter the address FROM where I would be sending BTCs.
But every time a send some BTC it seems to be sent out from a different address...
Should I use the one I always see when I say I want to "receive" some BTC? Is it my "main" address?

user61086: There is no from address in Bitcoin.
https://en.bitcoin.it/wiki/From_address
Each transaction reference an unspent output (UTXO) that was sent to you in a previous transaction.
About the address, you need to generate a new one for each payment:
https://en.bitcoin.it/wiki/Address

61079: <p>This website ask me to enter the address FROM where I would be sending BTCs.
But every time a send some BTC it seems to be sent out from a different address...
Should I use the one I always see when I say I want to "receive" some BTC? Is it my "main" address?</p>
 61086: <p>There is no <code>from</code> address in Bitcoin.
<a href="https://en.bitcoin.it/wiki/From_address" rel="nofollow noreferrer">https://en.bitcoin.it/wiki/From_address</a></p>

<p>Each transaction reference an unspent output <code>(UTXO)</code> that was sent to you in a previous transaction.</p>

<p>About the address, you need to generate a new one for each payment:
<a href="https://en.bitcoin.it/wiki/Address" rel="nofollow noreferrer">https://en.bitcoin.it/wiki/Address</a></p>
