Twilio has released Subaccounts, a tool for building multi-tenant web services that allows users to track usage of each user independently from one another, with all usage billed up to the main account. This feature is useful for powering several projects under one account and wants to divide up usage of phone calls and text messages, segmenting users into subaccounts for reporting and invoicing purposes. Subaccounts can be managed using the Twilio REST API and have their own account sids, phone numbers, and caller IDs, allowing for independent management without affecting other subaccounts. Additionally, features such as transferring numbers between subaccounts, suspending usage, and closing subaccounts are available to manage Twilio services more efficiently.