When you're using Terminal Service as a logon for your Windows server, you'll need this plugin when you have morde than 5 Terminal Service clients.

Which plugins are needed in which situations:
Situation 1)
Windows server with local database and LAN clients
Lan Clients => Multi Tier plugin

Situation 2)
Windows server with local database and both LAN and Terminal Service clients
Lan Clients => Multi Tier plugin
TS clients => TS plugin

Situation 3)
Windows server with local database and Terminal Service clients
TS clients => TS plugin

Situation 4)
Windows server with external database and LAN and/or Terminal Service clients
All Clients => Multi Tier plugin
TS clients => TS plugin