Multi-tenancy architecture (MTA) is often used in Software-as-a-Service (SaaS) and the central idea is that multiple tenant applications can be developed using components stored in the SaaS infrastructure. Recently, MTA has been extended where a tenant application can have its own sub-tenants as the tenant application acts like a SaaS infrastructure. In other words, MTA is extended to STA (Sub-Tenancy Architecture). In STA, each tenant application not only needs to develop its own functionalities, but also needs to prepare an infrastructure to allow its sub-tenants to develop customized applications. This paper formulates eight models for STA, and discusses their trade-offs including their formal notations and application scenarios.