Rectangular tubing is a hollow structural section, and therefore not pressure rated. Rectangular tubing sections are commonly welded together to form frames, especially where members experience loading in multiple axes.
Round tubing can be pressure rated or structural. Pressure rated round tubing is used to transport many different media, such as water, hydrocarbons, milk, or air. Round structural sections are often used to create columns, due to their uniform geometric characteristics and uniform strength across their cross-sectional axes.
Square tubing is a hollow structural section, and therefore not pressure rated. Square tubing is simply rectangular tubing with equal side width and height, and can be used for the same purposes.