The "symantec appcenter" ISO is the Mobility front end server, this is the main product installation.
The "tenant manager" is a separate and optional utility for multi-tenant deployments. Let say that a company wants to setup Mobility but have each geo manage their own independant tenant, while all connectining to a centralized server. Multi-tenancy allows for this, each "tenant" is independently managed as if it were its own installation. Or a consulting firm wishes to resell a cloud solution, multi-tenancy mode would fullfill this.