What happens if you set prune= to a value between 0 and 550?
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=&lt;N&gt;:


N &lt; 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 &lt; N &lt; 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.