Hello Kevin
When implementing OSPF, if there are many routers connected to the same broadcast domain, a DR and BDR election takes place. All routers in the broadcast domain create two neighbor adjacences: One to the DR and one to the BDR. There is also an adjacency between the DR and BDR.
When implementing IS-IS, there is a similar mechanism. The Designated IS (DIS) is elected and it creates a virtual router called a pseudonode. This pseudonode plays the corresponding role of the DR in OSPF. However, in this case, each router will create a single neighbor adjacency with this virtual device. Even the physical DIS creates an adjacency (internally) to the pseudonode.
So each IS-IS router doesn’t create an adjacency with every other router in the broadcast domain, but only a single adjacency to the pseudonode. Does that make sense?
I hope this has been helpful!
Laz