Why does the layer fail to show up in the Start Editing list in ArcMap?

The reason the layer fails to appear in the Start Editing list in ArcMap is that the data loaded in the map is from a read-only source. When data is sourced from a read-only format, ArcMap cannot initiate editing on that layer because it cannot modify the data itself. This restriction is inherent to the file type or database configuration from which the data originates, thereby preventing any editing operations from being conducted.

In ArcMap, only editable layers can be included in the editing session, specifically those that are in formats that permit changes, such as file geodatabases or shapefiles that are not set to read-only. If the data layer was indeed editable, it would have appeared in the Start Editing list allowing users to make the necessary edits and adjustments.

The other scenarios—such as a layer being unselectable, being in a different spatial reference, or needing a spatial index—do not directly affect whether the layer can be included in the editing session. While these factors could hinder other functionalities, they do not prevent a layer sourced from a read-only situation from being accessible for editing.