Note: This setting is only relevant to map mods. non-maps don't need to set it.

By default map mods will load the first alphabetically loaded map in the plugin in SP. But for server there is a convenience that can be provided to them (Nitrado will require this be filled in) by setting what map name is the entry map for your project. This will automatically set the entry map for singleplayer but it will NOT automatically set the map to load on servers. the file will be available in the mods "preview" folder though so that server admins and tools can refer to the file

Even with world partition there is the possibility for maps to have multiple levels and mod authors may generally want to set any one of them as the entry map.

Just set the name of the umap file without extension.
e.g. if this were for  The Island, we would enter TheIsland_WP and NOT TheIsland_WP.umap