The SFTP Server CDK L3 construct is used to deploy the resources for an SFTP Transfer Family domain inside an account/VPC.

SFTP Service Role - A role which can be assumed by the SFTP Transfer Family service in order to run the service and create logs within the account.
SFTP Server Security Group - A security group which defines who can connect to the SFTP Server. This SG will be bound to the network interfaces created for the server on the VPC.
SFTP Transfer Server - This SFTP Transfer server will be created within the account and bound to the specified VPC and subnets.