The following categories exist on the wiki, and may or may not be unused. Also see wanted categories.
- Bicycles (1 member)
- Bikes (1 member)
- Bilingual dictionaries (1 member)
- Biopharmaceuticals (1 member)
- Blockchain (2 members)
- Boekwinkeltjes.nl (1 member)
- Boilers (2 members)
- Brewing (1 member)
- Business (7 members)
- CAO (1 member)
- CAT tools (4 members)
- CIMAC (1 member)
- CINOP (1 member)
- CISA exam (1 member)
- COVID-19 (2 members)
- CPV (4 members)
- CROHO (1 member)
- Call centres (3 members)
- Cameras (1 member)
- Cannabis (1 member)
- Cardiagnostics.be (3 members)
- Carpets and rugs (1 member)
- Categories (1 member)
- Cedefop (3 members)
- Cement (1 member)
- Centrum voor Innovatie van Opleidingen (CINOP) (1 member)
- ChatGPT (1 member)
- Chemical elements (1 member)
- Chemistry (4 members)
- Chris P. Odijk (1 member)
- Cito (1 member)
- Civil Engineering (1 member)
- Cleanrooms (1 member)
- Coastal engineering (1 member)
- ComPsych (4 members)
- Combustion engines (1 member)
- Concordancers (2 members)
- Consilium (1 member)
- Construction (17 members)
- Council of the European Union (1 member)
- Crime (1 member)
- Crossword puzzles (1 member)
- Crypto (3 members)
- Czech (1 member)
- DSL (5 members)
- Danish (1 member)
- Diamonds (2 members)
- Dictation (2 members)
- Dictionaries (7 members)
- Digitising (0 members)