shibo - @Seve what does this mean?

you're using blind/buried vias

are you using the fanout solver

no

so these vias are a result of trying to route bgas without using fanouts?

no... i don't think the regular autorouter supports buried/blind vias so i'm wondering how you could possibly have them

they are vias that are only on some layers and not others

they are used during HDI/fanout

sometimes