salvadorc17 wrote:What you mean last resource
In easy terms, it means that the last modification you've made with the level editor (eg. editing a tile, placing a wall somewhere, changing a tile background) cannot be accepted because it will force some data (into the ROM image) to be pushed away (as far as I know, the Konami screen). So, imagine you are editing level 17 and after some editing you remember to save your process, if the warning "Last resource doesn't fit into its place" appears, it means that in some point you will need to undo your process, like re-doing a tile you edited with the graphic editor (leaving it in blank). This is why it is recommended, not only to constantly backup your mod, but also to constantly save your process to make sure you still have free space to continue editing.
If you decide to go on saving ignoring the warning, then the Konami screen will disappear. I don't know if something else in the .rom disappears, but the Konami screen does for sure.
So, with the level editor's new version, the new tiles or editings you make that probably would push some data away, as David said, will be placed somewhere after the sprites "section" in the .rom file.