Manifests are the new files that come with entrenchment, however for some reason they don't provide you with these new additions when you download the Forge Tools 3 kit. They appear as obscure looking files in your entrenchment sub folder - located in your installation folder for the game. There are 6 of them in total and editing them is quite straightforward. You just need to right click on the "Galaxy.Manifest" file that you stated above, then select notepad to open the file. It merely lists the names of the galaxy maps (defaults) that are available. When you are making your own maps with the intent of using them in game, you would need to update this file with the name of your maps using the format presented in that file whilst also updating the number count before saving. Of course this is only if you want to add maps to your default game. If you are only adding them as part of a mod then you would need to copy the default Galaxy.Manifest file in the Entrenchment installed folder to your mod folder, before editing, in order for the new maps to show up upon mod activation.

Hope this helps!