What does 2+4+2 mean in PCB buildup with 8 layers?

When it comes to PCB buildup with 8 layers of 2+4+2 as an example,
what does 2+4+2 stand for here?

Thanks a lot for your help.

The + normally refers to the filler, I think. So your example would be made with a 2 layer PCB, a filler, a 4 layer PCB, another filler and finally another 2 layer PCB. The fillers are simple blank PCB material.

Keith