What are the main differences of this two types of flights?
Sub charter is not used nowadays as a word expression. It's now called wet lease or ACMI. An ACMI specialist airline does not fly its own regular schedule, but usually flies for other airlines on shorter/longer notices (couple of flights or whole schedule). When WOW air started in Iceland, its schedule was flown by a Lithuanian ACMI operator. HiFly flies for Condor its service from Munich this winter, because Condor's 767s are undergoing heavy maintenance. Thomas Cook of UK is a permanent wet leaser to Condor (they are in the same holding). They provide aircraft and pilot crew, but Condor does fuel, maintenance and cabin crew. In the HiFly example above, HiFly provides full ACMI (aircraft crew maintenance insurance) for Condor.