The NewGRF specs are the official technical documentation and information about the grf (graphics ressource file) format which is shared and used by both, OpenTTD and TTDPatch. This wiki is meant as both, spec manual for NewGRF authors as well as for programmers which work on the code side, implementing, amending or verifying and fixing features.
As the grf format is a perpetually evolving format, the minimum (and, if needed maximum) programme versions which support the availability of a certain feature in OpenTTD or TTDPatch are usually indicated as well as availability in different grf format versions.
When amending the wiki be concise and to the point and clear as suitable for a technical documentation. Brief examples are welcome, elaborate examples better go to the tutorials.