What spatial reference will ArcMap assign to a layer with no spatial reference added first?

When a layer is added to ArcMap that does not have a defined spatial reference, ArcMap will assign it a spatial reference status of 'Unknown.' This means that the software recognizes a lack of defined spatial characteristics for that layer, indicating that there is no information available about how to place the data correctly in geographic space.

In practical terms, this can lead to issues when working with geographic data since the layer cannot be accurately placed or used in conjunction with other spatial layers that have defined coordinate systems. It serves as a reminder to the user that they need to assign a proper spatial reference to ensure the layer displays correctly and aligns with other spatial data in their project.

In contrast, some options might suggest that the system defaults to a specific coordinate system or retains the last used one. However, without an assigned spatial reference, it cannot inherit, default, or remain set until other layers are added; instead, it is simply marked as 'Unknown' until manually addressed by the user.