From discussion in the group and the text under "Publishing Target", the name should be "Base Asset". Even though it is mentioned there that it is sometimes referred to as Source Asset, it may be confusing to find two different wordings in the same document.
In section sec01_FileFormatsAndAssetStructure the base asset is called "source asset".
In section sec04_UVCoordinates the base asset is called "source model".
In the summary it is only referred to "Source Model" (https://github.com/KhronosGroup/3DC-Asset-Creation/blob/main/asset-creation-guidelines/RealtimeAssetCreationGuidelines.md).
From discussion in the group and the text under "Publishing Target", the name should be "Base Asset". Even though it is mentioned there that it is sometimes referred to as Source Asset, it may be confusing to find two different wordings in the same document.