“normally”, there is only tribble and holodeck. Tribble is where new items and missions are placed before going to the holodeck (the main ‘real’ or ‘live’ server). However, when a new MAJOR update is being tested (like season 6), and requires an extended testing period, they activate a third server – Redshirt. Redshirt takes over for Tribble in testing the weekly updates that go live, while Tribble gets turned into the season testing.
Holodeck is main server.
Tribble is season 6 test server where our starbase is.