This T-shirt size pricing model is based on RPS (Request Per Second) requirements of the projects. Any project which will be onboarded to Global Shared APIM will choose their RPS based on which they will be charged per Quarter. If the project's RPS requirement increases, they can inform the same after which they will be charged for the next higher size.
Note: All the billing will be calculated from 1st to end of the month. Any changes to RPS will be accommodated in the next billing cycle. However, project's RPS requirement will not have to wait for the next billing cycle.