Parameters of certain events are expected to be indexed (e.g. ERC20 Transfer/Approval events) so that they’re included in the block’s bloom filter for faster access. Failure to do so might confuse off-chain tooling looking for such indexed events. (see here)
- Event Parameters -> Not Indexed
- Indexed -> Topics
- Not Indexed -> Data
- ERC20 Specification
- Faster Lookup
- Add
Indexed-> Critical Parameters