It can be hard to know which way your poster should be oriented by the instructions you get from the conference. Graphic designers are pretty good about giving you the width of the poster first, so that a portrait oriented poster is given as 36×48, while a landscape poster is (correctly at least) given as 48×36.
The problem is that a lot of people aren’t really familiar with the convention. So they read that the conference wants a 36×48 poster, and they grab a template for a 48×36. The rule I use is that if somebody gives you dimensions of 48×36, they know about the convention, and if they give it as 36×48, who knows what orientation is intended!
Portrait oriented posters are very common in Europe, and we do see them used in the US from time to time. The best advice I can give you is to be really sure about the space you are being given, because it is truly terrible to arrive there with a poster that doesn’t fit on the board!
One other thing you can generally be sure of is that if the space given is 48×96 inches, it is probably really a landscape poster space. The conference won’t ask you to get on a stepladder to put it up!