Yes it depends on what node you are using.
For example if you want to use the Category as a string for input into a Category.ByName Node then you would have to NOT call it “Rooms” you would have to call it “OST_Rooms”

To confuse you further the Category names shown in the Category selection node do NOT use the OST prefix.

Confused? I was. Hopefully this image will sum it up for you.

The real question is how was the general DynamoBIM Community supposed to know that the Built in category name should use the “OST_” prefix in some cases and not in others… or for that matter that the prefix was even called “OST_” …. really….. 
As I understand it, “OST_” was a “hold over” from API language that DynamoBIM relies on. But at the very least the names should be consistent… IMHO 
Dont get me wrong I love DynamoBIM and it has made my Revit Life very productive. I am just trying to clarify the “very” confusing DynamoBIM naming conversions.