Semantics vs. extensions

[lilleyse]

One pain point has been when to use semantics vs. extensions.

• There are some semantics that are purely informational like ID, NAME, and DESCRIPTION. Most user defined semantics fall under this category. These should remain semantics.
• There are some semantics that affect behavior like TILESET_CRS_GEOCENTRIC that should probably have been extensions.

Then there's this third category of semantics that are convenient to encode as binary metadata properties, e.g. in a property table or property texture, in a subtree or in a glTF, that also affect runtime behavior. Examples of this are:

• TILE_MINIMUM_HEIGHT, TILE_MAXIMUM_HEIGHT, etc for implicit tiling
• WATERMASK property texture semantic for https://github.com/CesiumGS/cesium/tree/3d-tiles-terrain

For this third category it could make sense to have a hybrid approach where you have both an extension and semantics. The extension would be a simple README (no schema) that defines the new semantics and describes their behavior and you would add it to extensionsUsed and/or extensionsRequired.