Definition
An exemption is a permission to keep using a restricted substance in one narrow application, granted because the industry has no workable substitute for it yet. It is the safety valve that stops RoHS from banning materials before replacements exist.
Exemptions are not open-ended. Each one is tied to a specific use, for example a particular kind of solder or a sensor material, and each carries an expiry date. When that date approaches, the exemption has to be reviewed and renewed, otherwise it lapses and the substance becomes restricted in that use as well.
RoHS keeps two lists. Annex III covers exemptions that apply broadly across equipment categories. Annex IV covers exemptions specific to medical devices and monitoring and control instruments. The full catalogue is set out in the RoHS exemptions reference.
An exemption you rely on can expire. Track the expiry date in your technical documentation, because once it lapses the substance is restricted again.
Note: general educational information, not legal advice. Check the official source before relying on it.