user62184: In a bitcoin transaction the previous unspent transaction outputs are referenced.
How are these referenced?

Do outputs have unique reference or maybe as
unique transaction reference X output recipient's address (its public key)


user5406: Each transaction output is uniquely identified by an outpoint. Outpoints consist of

txid: The unique transaction ID of the transaction that created the output. The txid is the 32-byte sha256d hash of the transaction. 
index: The output index (vout) that marks the position of the output in that transaction. The output index starts counting at zero and is a 4-byte unsigned integer.

As an arbitrarily picked example, the outpoint 
e06a3193c6ac49b8dd862baf9bd3561ffe86d65b6e8a9e73ebe1dc91e3ca2978:1

refers to the second output of the transaction e06a3193c6ac49b8dd862baf9bd3561ffe86d65b6e8a9e73ebe1dc91e3ca2978 worth 0.001 BTC which was sent to address 1La27vpuDDr9NR1aZ6GpxoJyNYmHT62nRb.

62184: <p>In a bitcoin transaction the previous unspent transaction outputs are referenced.</p>

<p>How are these referenced?</p>

<blockquote>
  <p>Do outputs have unique reference or maybe as</p>
  
  <p><code>unique transaction reference</code> X <code>output recipient's address (its public key)</code></p>
</blockquote>
 5406: <p>Each transaction output is uniquely identified by an <a href="https://bitcoin.org/en/developer-reference#outpoint" rel="nofollow noreferrer"><strong>outpoint</strong></a>. Outpoints consist of</p>

<ul>
<li><code>txid</code>: The unique transaction ID of the transaction that created the output. The txid is the 32-byte sha256d hash of the transaction. </li>
<li><code>index</code>: The output index (<code>vout</code>) that marks the position of the output in that transaction. The output index starts counting at zero and is a 4-byte unsigned integer.</li>
</ul>

<p>As an arbitrarily picked example, the outpoint </p>

<pre><code>e06a3193c6ac49b8dd862baf9bd3561ffe86d65b6e8a9e73ebe1dc91e3ca2978:1
</code></pre>

<p>refers to the second output of the transaction <a href="https://www.smartbit.com.au/tx/e06a3193c6ac49b8dd862baf9bd3561ffe86d65b6e8a9e73ebe1dc91e3ca2978" rel="nofollow noreferrer"><code>e06a3193c6ac49b8dd862baf9bd3561ffe86d65b6e8a9e73ebe1dc91e3ca2978</code></a> worth 0.001 BTC which was sent to address <code>1La27vpuDDr9NR1aZ6GpxoJyNYmHT62nRb</code>.</p>
