Use it when you create a custom field that will contains a cron expression:
<field id="cron_expression" translate="Cronexpression"
type="text" sortOrder="100" showInDefault="1" showInWebsite="0" showInStore="0">
<label>Cron Tab String</label>
<comment><![CDATA[
<pre>
* * * * *
| | | | |
| | | | +---- Day of the Week (range: 0-6, 1 standing for Monday)
| | | +------ Month of the Year (range: 1-12)
| | +-------- Day of the Month (range: 1-31)
| +---------- Hour (range: 0-23)
+------------ Minute (range: 0-59)
Example: 0 0 * * * Daily at midnight
</pre>
]]></comment>
<validate>required-entry cronexpression</validate>
</field>