The Status Out pin controls the function of the INB/INB+ and the SI/INB- inputs. When SO is tied directly to Vcc, these two pins form INB+ and INB-, a differential line-receiver serial-data input pair. When SO is tied to a normal TTL-load without any pull-ups, then the first pin becomes a single-ended serial line receiver while the second pin becomes a single-ended Status Input pin.
This is done with a sensor circuit connected to the SO pin. It senses how this output is externally connected and changes the mode of INB/INB+ and SI/INB- as a result. This sensory circuit is always active and so it is possible to change the status of SO during operation.
If, for example, there is a pull-up during the start-up or configuration of the system, but that pull-up disappears when the system is loaded, then the state of the two pins INB/INB+ and SI/INB- will change from being a differential pair to two single-ended signals.