Special category data refers to personal data that is regarded as particularly sensitive. This kind of data is subject to stricter processing laws due to the increased levels of harm that a disclosure of special category data could do to the individual involved. Forms of special category data include information about health, racial or ethnic origin, political opinion, trade union membership, and gender and sexuality.
In addition to identifying a legal basis for processing special category data organisations must also identify an additional ‘condition for processing’. The ICO sets out what these are here: Special category data | ICO. Organisations that are processing special category data need to ensure that they take extra care and document how they are ensuring the safety of the data.