I'll try my best to explain, altough i'm very poor at it and not sure if i got it right either.
To make a new mapping, click the "tri" tab and in the bottom left duplicate the existing mapping and right click on the color box to rename it something else. Then select the triangles you want to have this mapping and left click on the color box to assign them for it.
Mappings can have any name you want, apart from the lights and body color options if necessary. For all of the available lights there are already preset names to choose from when you're renaming the mapping, but for body colors it's commonly M1_x to M4_x(main). to create copies of these colors for other specific places you want to have adjustable color in you can use C1_x to C4_x(copy). If you want only one adjustable color then you only use M1 and C1 mapping for every place, if two separate colors you define the places that'll have the 2nd color option as M2 and C2, and so on.
You will see this mapping on the "mapping" tab after this. It can be assigned a cutout, but as this doesnt exist yet you need to create a new cutout in the "cutout" tab. Click on "add new cutout" and name it whatever you want. You can now assign this cutout for specific mapping back at the mapping tab by clicking on the cutout paragraph and choosing the new cutout from a list on top right. You can also change the material properties in the cutout page by clicking on the material paragraph.
For loading textures into the editor, you have the "page" tab. Each texture is basically a page, and these have to be .png files and located in the editor/data/png folder. When you click on "add new page" you will be greeted by the list of textures that are inside data/png, choose the texture you want and repeat for all the textures you need for your model.
Assign these pages in the cutout tab by clicking on the page paragraph of the corresponding cutout. After that, you can adjust the mapping and positioning of a texture in the "mapping" tab.
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Back on topic though, i am very biased when i say this but would love to see an engine similar to 155 V6 TI or Honda's C or J engines, instead of the poor excuse of a V6 by GM
It is not too much an issue though, i'll propably end up driving this regardless of what engine is in it