The directory replicates all partitions that are common between the two servers. The site link allows the administrator to assign the cost and transport for replication. This procedure defines parameters for replication. The cost is an arbitrary value that is selected by the administrator to reflect the speed and reliability of the physical connection between the sites. When you lower the cost value on the link, the priority is increased.
Site links have a replication interval and a schedule that are independent of the cost.
The cost is used by the KCC to prefer one site link path over another. If a site link has more than two sites, all of the sites in the site link are considered connected in a NxN fully connected star topology. The KCC uses site links to decide which sites to link with connections. Without site links, the KCC has no information about the sites that are reachable on the network and does not know the relative costs of the WAN links between the sites. You should add at least enough site links so that every site is transitively linked to every other site.