1. Classified by reinforcing materials (most common method):
 
- Paper-based substrates: FR-1, FR-2, FR-3
- Epoxy glass cloth substrates: FR-4, FR-5
- Composite substrates: CEM-1, CEM-3
- HDI materials, RCC
- Special substrates: metal-based, ceramic-based, thermoplastic, etc.
 
2. Classified by resin:
Phenolic resin boards, epoxy resin boards, polyester resin boards, BT resin boards, PI resin boards.
3. Classified by flame retardancy:
Flame-retardant (UL94-V0, UL94-V1) and non-flame-retardant (UL94-HB).
 
Any other common or niche substrates to share? Drop them in the comments!