Citrix Session Host and Controller

Citrix Session Host - Worker
Citrix Controller - Zone Data Collector

XenApp 6.5 offers new roles, including a worker role (session host only) and a controller role. The default role for a XenApp server is a controller role. The controller server is responsible for farm management tasks while the only task of the session-only servers or workers is to host user sessions.
A farm requires at least one server to act as a controller, which can host the XML and Zone Data Collector roles. A worker cannot perform the role of a data collector, nor can it trigger elections.
There might be times when you need to determine the role of a particular XenApp server in the farm.
To detect if a XenApp 6.5 server is a worker or a controller, complete the following procedure:

1. Open the AppCenter Console.
2. Navigate to the Servers section.
3. Click the server you want to check.
4. In the Information tab note the value for the Session-host Mode field. Following are the values for this field:
   • Disabled: Indicates that this server is a controller.
   • Enabled: Indicates that this server is a worker.

You are correct: Session Host Mode: Disabled means the server is a "Controller"

Your second statement is not correct........ 

Controller = can do anything.. can be a dedicated XML Broker or Zone Data Collector or Host apps and the first server in the farm

has to be a controller (think: if you only have a 1 server farm - it must be an XML Broker, Zone Data Collector as well as host apps for your users). 

Worker (Session Host Mode: Enabled) = Only works to server user by hosting apps.  Cannot be used for controller functions (XML/ZDC) without remove/re-add to farm and specifying new role. 

By default ... XenApp server are all controllers when you build them ...unless you specify Session Host Only. If your farm of (any number of) XenApp servers are default installs then they are all controllers and can perform any function (XML/ZDC/App host) - doesnt affect anything other than maybe a slight performance hit.

The  second method to detect if a XenApp 6.5 server is a worker or a controller is to check the server registry in the following location:

HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Citrix\IMA\RUNTIME

For the DWORD named WorkerRole, a value of 0 (zero) indicates that the server is a controller and a value of 1 indicates that the server is a worker.

Switching Between a Worker or a Controller

You can switch between a worker or controller with the XenApp Server Role Manager:
To  switch between a worker or a controller, complete the following procedure:

1. Start the XenApp Server Role Manager.
2. Remove the server from the farm.
3. From the XenApp Server Role Manager click Configure.
4. Click Add this server to an existing server farm to add the farm:
5. Select the appropriate database and shadowing options.
6. On the XenApp Server Configuration, enable the Data Collections Options, as shown in the following screen shot:

7. Note the session host only dialog. Session-host indicates that the server is a worker.

Source: I took this information from support.citrix.com