The Heroku add-on can be applied to only one project on Semaphore, the one which you configure after provisioning the add-on. The add-on pricing is designed with that limitation in mind, and varies based on number of boxes available to that project for parallel jobs.
A project created via Heroku add-on can belong to any organization account on Semaphore, but it cannot use the box resources that otherwise belong to that organization account — only the boxes allocated to it by according to the Heroku add-on.
Billing of the Heroku add-on is managed by Heroku and incorporated in the invoice of your Heroku application.
Accounts created directly on semaphoreci.com can accomodate an unlimited number of projects, and the billing is managed by Semaphore. Boxes allocated by the account can be shared among all projects that belong to that account.
You can see the current status of your Heroku add-on plan and usage in project settings > Heroku Add-on tab.
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