Development tree tiers and branches
Trees expand unlocked services with options arranged in tiers and, for some services, branches; Development Points unlock further nodes.
How it works
When a milestone unlocks a service, its basic buildings and features and a corresponding tree become available. The first tier's basic node is automatically unlocked with the service. Later tiers offer more advanced options and cost more Development Points. Branches require purchasing earlier nodes on that branch, but a player need not purchase every node in a tier to progress elsewhere; the source gives International Airport as accessible without first unlocking water, tram, or subway transport if enough points are available.
Read the supporting references
Check the edition and source version for patch-specific details. Guide methodology.