Broadly speaking, at Caledonian (when owned by BA) the Tristars were used short haul to the Mediterranean, on traditional Airtours operations, and the DC-10s were used on long haul holiday flights, principally to Florida and the Caribbean. It varied each season but this was the general trend. Mostly both fleets operated from Gatwick, with minor departures from elsewhere. As had been longstanding with the Airtours/Caledonian holiday flight operations, standing in for main line operations from Heathrow in the off season was part of the way of keeping winter utilisation up.
Likewise the mainline BA Tristars operated from Heathrow and the DC-10s, after being absorbed from B. Cal, from Gatwick, although there were odd exceptions for short intervals both ways.