Tubes 1-4 are standard tubes that are loaded from the "Main Rack". These can fire anything but wireguided weapons (TEST-71s, UUVs etc)
Tubes 5 and 6 are loaded from their respective racks (#5 Rack etc) and can launch any weapon and has wire guide capablity.
The P is for the tube's Presets screen (RTE, Depth, Speed etc) the W (on 5&6) is for the Wireguide screen for that tube.
NOTE: TEST-71s should only be loaded in the Tube 5&6 racks NOT THE MAIN RACK! They cannot be loaded from the main rack to tubes 5 and 6 (SCS
Really should remove them from that part of the loadout screen or make them capable of fire and forget use
) Also the PLAN TEST-71s once enabled and locked on to a target cannot be pre-enabled and redirected to another target, they can only be shutdown or pre-enabled (which just prevents the weapon from exploding).