Classic custom fields were the initial version of custom fields in Manager. This feature is now obsolete and has been replaced with an improved custom fields system.
We strongly advise against using classic custom fields. The new custom fields system offers better functionality and improved performance.
Learn more about the new custom fields: Custom Fields
To convert your classic custom fields to the new system, click the Upgrade button located in the bottom-right corner of the Classic Custom Fields screen.
For detailed upgrade instructions, see: Classic Custom Fields — Upgrade