user5406: Today I saw a question about what Bitcoin Core would do if you set prune=1. The documentation on bitcoin.org states: 

A value of 0 disables pruning. The minimal value above 0 is 550.

Apparently, the log upon setting prune=1 states:

2017-03-11 10:51:03 Block pruning enabled.  Use RPC call pruneblockchain(height) to manually prune block and undo files.

Will Bitcoin Core prune to the minimum disk usage now, or does it just enable manual pruning?
The asker was running Bitcoin Core 0.14.0, if it makes a difference.

user208: From the Bitcoin Core v0.14.0 release notes:

Bitcoin Core has supported automatically pruning the blockchain since 0.11. Pruning the blockchain allows for significant storage space savings as the vast majority of the downloaded data can be discarded after processing so very little of it remains on the disk.
Manual block pruning can now be enabled by setting -prune=1. Once that is set, the RPC command pruneblockchain can be used to prune the blockchain up to the specified height or timestamp.

So this means that if you set -prune=1, the node will act as a pruned node (not advertize blocks to other nodes), but not actually delete anything. Instead, it enables the pruneblockchain RPC which you can use to manually prune block files after you're done with them. The reason for this is that you may need the block files in some external application, and don't want Bitcoin Core to go delete them from under you.
I assume the documentation on bitcoin.org is out of date.

user5406: It turns out that prune=1 is a new special case which enables manual pruning. For values of N from 2 to 549, prune=N shows an error.
Bitcoin Core parses configuration arguments in the init.cpp. Depending on the value of prune=<N>:

N < 0 – returns the error: "Prune cannot be configured with a negative value."  
N = 1 – Activates manual prune mode as described in Pieter's answer. It causes Bitcoin Core not to advertise blocks anymore and enables the pruneblockchaincommand, but it doesn't automatically prune the blockchain data.
1 < N < 550 – returns the error "Prune configured below the minimum of N MiB.  Please use a higher number."
550 ≤ N – Sets N as the limit of MiB available for block data.


5406: <p>Today I saw a question about what Bitcoin Core would do if you set <code>prune=1</code>. The documentation on <a href="https://bitcoin.org/en/full-node#reduce-storage" rel="noreferrer">bitcoin.org</a> states: </p>

<blockquote>
  <p>A value of <code>0</code> disables pruning. The minimal value above <code>0</code> is <code>550</code>.</p>
</blockquote>

<p>Apparently, the log upon setting <code>prune=1</code> states:</p>

<blockquote>
  <p>2017-03-11 10:51:03 Block pruning enabled.  Use RPC call pruneblockchain(height) to manually prune block and undo files.</p>
</blockquote>

<p>Will Bitcoin Core prune to the minimum disk usage now, or does it just enable manual pruning?</p>

<p>The asker was running Bitcoin Core 0.14.0, if it makes a difference.</p>
 208: <p>From the <a href="https://bitcoin.org/en/release/v0.14.0" rel="noreferrer">Bitcoin Core v0.14.0 release notes</a>:</p>

<blockquote>
  <p>Bitcoin Core has supported automatically pruning the blockchain since 0.11. Pruning the blockchain allows for significant storage space savings as the vast majority of the downloaded data can be discarded after processing so very little of it remains on the disk.</p>
  
  <p>Manual block pruning can now be enabled by setting <code>-prune=1</code>. Once that is set, the RPC command <code>pruneblockchain</code> can be used to prune the blockchain up to the specified height or timestamp.</p>
</blockquote>

<p>So this means that if you set -prune=1, the node will act as a pruned node (not advertize blocks to other nodes), but not actually delete anything. Instead, it enables the <code>pruneblockchain</code> RPC which you can use to manually prune block files after you're done with them. The reason for this is that you may need the block files in some external application, and don't want Bitcoin Core to go delete them from under you.</p>

<p>I assume the documentation on bitcoin.org is out of date.</p>
 5406: <p>It turns out that <code>prune=1</code> is a new special case which enables manual pruning. For values of <code>N</code> from 2 to 549, <code>prune=N</code> shows an error.</p>

<p>Bitcoin Core parses configuration arguments in the <a href="https://github.com/bitcoin/bitcoin/blob/30ff3a2fc95ca6e5b87e6d0f86e54ed9069dae5e/src/init.cpp#L973" rel="noreferrer">init.cpp</a>. Depending on the value of <code>prune=&lt;N&gt;</code>:</p>

<ul>
<li><code>N &lt; 0</code> – returns the error: "Prune cannot be configured with a negative value."  </li>
<li><code>N = 1</code> – Activates manual prune mode as described in <a href="https://bitcoin.stackexchange.com/a/52050/5406">Pieter's answer</a>. It causes Bitcoin Core not to advertise blocks anymore and enables the <code>pruneblockchain</code>command, but it doesn't automatically prune the blockchain data.</li>
<li><code>1 &lt; N &lt; 550</code> – returns the error "Prune configured below the minimum of <code>N</code> MiB.  Please use a higher number."</li>
<li><code>550 ≤ N</code> – Sets <code>N</code> as the limit of MiB available for block data.</li>
</ul>
