Under 'Flying the campaigns', the little printed manual that comes with the Abacus 'Mosquito Combat Europe' says this of the TAC, which corresponds exactly with my experience of it:
'Unlike the 'radar' in CFS2, the tactical display is not a 'god's eve view' of the action. In fact it will only show you want your pilot could actually see.
This means that aircraft flying in heavy cloud won't show on the display. Likewise ground units obscured by cloud or too small to be seen at distance won't be shown on the display. But it gets even more complicated than that.
The tactical display is turned on or off using SHF+T. It represents what your pilot can see given his current health and vision. The health (fatigue) and vision of your pilot will change from mission to mission, and can be increased by spending 'skill points'.
You acquire skill points by flying successful missions and by making kills. Before you fly your next mission you should check the status of your pilot and apply any skill points you have acquired'