Runs when a unit instance is being made. In practise that can be a unit that just got spawned or a unit that started being built.
In unsynced mode the callin gets invoked whenever an instance of your units is made. In synced mode it invokes for any unit in the game.
function widget:UnitCreated(unitID, unitDefID, unitTeamID, builderID) Spring.Echo("Unit "..unitID.." from team "..unitTeamID.." just got created by "..builderID) end