Workflows in Sitecore reflect how editors in an organisation – or tenant - work with the content. In a multi-tenant scenario, two autonomous tenants (see Multi-site and multi-tenant) with different project layer modules, different content architectures, page types, and sites will often need different workflows assigned. Therefore, the actual Sitecore workflow definitions belong in the Project layer modules.
To cater for flexibility across sites and projects, workflows are always assigned to Page Type or Datasource templates (see Template types). Therefore, workflows are managed in a Project layer module.
Any custom actions or other business logic extensions to the Sitecore workflow engine will belong to a foundation or feature layer module.