Fleetwood-Whole house fans?

[Craig in NorCal]

Been looking at dozens of new models here in Northern Cal and it seems all of the Fleetwood models (Doubles & Triples) have a switch on an inside wall with "Fan must be on at all times" tag on it. Haven't noticed it on any other brands. What is it that Fleetwood needs to "VENT" out of their homes? Are they covering for a past lawsuit or what?
I am very impressed with the professionalism & knowledge on this site & will have many ??? in the future!

Thanks, Craig

[mac]

Moisture. MH are sealed tightly and moisture is removed by ventilation. Other ways to do this include a vent into the furnace (only vents when furnace is on) and small vents built into each window. Much more a problem in the South where there is lots of warm, moist air. MH builders don't generally use a vapor barrier of plastic over the interior of the studs. Instead, the vinyl-coated sheetrock is to do that. I am sure others with more knowledge than I will chime in.