According to NewGRF spec rules for Action 8, all GRFIDs with ranges from FF 00 00 00 to FF FF FF FF are exclusively reserved for System functions.
| GRFID | Purposes |
|---|---|
| FF 00 00 00 .. FF FF FF FF | reserved |
| FF "OT" 00 .. FF "OT" FF | extra base GRFs |
| FF "OT" 01 | OpenGFX |
| FF "OTD" | OpenTTD (extra) base graphics: openttdd.grf (OpenTTD 0.6 - 1.0) |
| FF "OTT" | OpenTTD (extra) base graphics: openttd.grf (OpenTTD >= 1.1) |
| FF "OTW" | OpenTTD (extra) base graphics: openttdw.grf (OpenTTD 0.6 - 1.0) |
| FF FF FF FF | TTDPatch (extra) base graphics |