<var>
A variable or placeholder the reader is expected to substitute.
Baseline: Widely available · Inline text semantics
When to use it
In command synopses and math: 'replace <var>file</var> with your output path'.
When not to
Not for italics — its meaning is 'this is a placeholder', nothing else.
Example
sx --out file input…<pre><code>sx --out <var>file</var> <var>input</var>…</code></pre>