This happens very often, probably more often in RME than others though. Select a face-based component to put in the model, start clicking away and then realize that it isn't hosted like you hoped it would be. Face-based families cannot be preset to prefer a certain face. Revit will decide that a Vertical Face is what you want by default. You have to remember to change it to one of the other two choices, Face or Workplane. It would be GREAT if we could preset a preference, not yet however.
Regular content, like a desk or a chair, doesn't offer a choice unless they have been changed to be Work-plane based. If that's happened then you get a choice between a Face and Workplane, like below.
You need to be careful with those choices if you have linked Revit models. The Face that Revit finds may be something in the link instead of the Level of your host project. If you select a family and find the associated work plane shows a linked project file you've got some "fixin" to do. Not hard to fix, just click the Edit Workplane button and choose the intended workplane, such as the Level it is on.
Face-based families offer you three choices and Vertical is the default one you'll get.
Placing content you have to be vigilant and notice which kind you are dealing with. If you don't change it you might get something like this, where your air terminal is hosted on the Vertical Face, or in as in the image below on the "vertical face" of the ceiling grid, uh oh!