in MongoDB 3.6 is introduced the concept of a seed list that is specified using DNS records, specifically SRV and TXT records. You will recall from using replica sets with MongoDB that the client must specify at least one replica set member (and may specify several of them) when connecting. This allows a client to connect to a replica set even if one of the nodes that the client specifies is unavailable
You can see an example of this URL on a 2.2.12 or later connection string
Note that without the SRV record configuration we must list several nodes (in the case of Atlas we always include all the cluster members, though this is not required). We also have to specify the ssl and replicaSet options
With the 3.4 or earlier driver, we have to specify all the options on the command line using the MongoDB URI syntax.
The use of SRV records eliminates the requirement for every client to pass in a complete set of state information for the cluster. Instead, a single SRV record identifies all the nodes associated with the cluster (and their port numbers) and an associated TXT record defines the options for the URI.
check the Reference