For the time being until the documentation is updated, you can think of large number support as being for numbers well outside the range that you could edit with a rotary encoder.
For example lets say you needed something that represented all frequencies from 0 to 1Mhz, you could not present that with a rotary encoder. Instead, with large fixed number you can edit those one digit at at time, and the are stored in BCD format of up to 12 digits.
Here's a few examples for the time being, better docs coming soon.
LargeFixedNumber* num = menuItem.getLargeNumber();
// To read the latest as a floating point
float flt = num->getAsFloat()
// To read the latest as a whole and fraction
bool neg = num->isNegative()
uint32_t whole = num->getWhole()
uint32_t fr = num->getFraction()
// to set a new value from a float
num->setFromFloat(value);
// to set as a whole and fraction
num->setValue(whole, fraction, isNegative);