What happens when feature classes within the same feature dataset are defined with differing coordinate systems?

When feature classes within the same feature dataset are defined with differing coordinate systems, the primary consequence is that the feature dataset is invalid and cannot be created. A feature dataset is designed to hold feature classes that are spatially related and share the same coordinate system, allowing for seamless spatial analysis and topological relationship management. If feature classes within that dataset have different coordinate systems, it leads to inconsistencies and potential errors in spatial data analysis, hence the system will not allow the creation of that feature dataset.

In contrast, the other options fail to address the fundamental requirement of coordinate system uniformity for feature datasets. Automatic conversion of differing coordinate systems may sound pragmatic, but it does not apply in this scenario since the system requires a consistent coordinate reference to maintain data integrity. Retaining only the first defined class also undermines the principle of organizational structure within the dataset. Lastly, saying coordinate systems have no effect is inaccurate, as they are vital for spatial relationships and data interoperability. Understanding this is crucial for effective GIS management and spatial data usage.