I got stuck in part where I should prepare uv-map the way that if someone else looks at it, he can easilly see which part of mesh is at which part of uv-space. I will go with tank as example again. Let's say, I would try to place uv-islands of the tank body on the bottom part, while uv-islands of his turret would be located at top part. Sounds easy but, I'm not sure how should I go about it. I'm working in Blender, so from as much I know for now, even if I've placed uv-islands where I want manually, margin between them would stop existing, and it would cause problems for texture padding. "Pack Islands" is going to place them back randomly on the uv-space and, I never heard about inside tool or addon for Blender that could at least tell user about the distance between 2 uv-islands. Or at least try to "pack islands" while trying to keep their position.
I'm aware that explaining it could take a lot of your time, so I would apprecipate even some recommendations for a tutorials (even paid ones) that are showing and explaining, how it is done.
But nowadays, with 3d painting with realtime feedback being the norm, this is all irrelevant. It's more about packing to get the most resolution out of your textures as possible, rather than packing to resemble your model.
Of course there is nuance to that, as not every asset is the same. But a clear unwrap is always a good bonus.
One easy thing to do is to group the UVs of various parts of the model as 1:1 squares and 1:2 rectangles. From there the overall assembly becomes very easy, and readable. Another bonus is that it allows to chop up the UVs very easily post-bake if there is ever a need to split up the models for optimization. For instance if the head of a character is fully contained within the top left quarter of the layout, then turning that head into an individual asset is just a matter of cropping the textures and scaling up the UVs 2x from the top left corner to fill up the full UV square. A huge time saver when it happens.
Now whether or not the above does apply to the example of a tank model is a topic in and of itself.