What are the CONT and FLT modes of the 737 start switches?
The 737 has two separate engine-igniter systems (each of which has igniters on both engines).  Each engine has four possible igniter modes (except for earlier-model Originals, which have five) - in clockwise order 'round the knob, they're GND, OFF, CONT (or, for early Originals, L IGN and R IGN), and FLT.


CONT mode turns one of the igniter systems on (which igniter system is used depends on the position of a separate switch), and is used for takeoff and landing.


GND mode on an Original or Classic, or on an NG that's on the ground, also uses a single igniter system, and is used for ground-starting the engines.

FLT mode turns both igniter systems on, the better to help keep the engines operating under adverse conditions, or maximise the chance of a successful midair relight.  In the real world, FLT mode is used only in conditions posing a significant risk of engine flameout (for instance, extreme turbulence or very heavy rain), or if a flameout has already occurred and one or both engines need to be restarted in flight - not for high-altitude cruise flight in general.


On an NG, GND mode is another option for inflight restarts, in which case it uses both igniters (unlike how it operates for ground starts).

Your simulator might be confusing FLT mode with OFF mode (neither igniter system operating), which is the setting normally used in flight (as combustion in a gas-turbine engine is generally self-sustaining unless the engine, or its fuel or air supply, is severely abused).


Additionally, on an NG, the EECs can automatically switch both igniter systems on (if an engine flames out) or off (if bad things happen during ground-starting), no matter what the position of the ignition knobs.