Triangle
Goto says "the triangle refers to viewpoint; I had it represent one's head or direction and made it green." Thematically then, it would make sense for game developers to use this button for players to change the viewpoint of the game, such as in sports games where several camera angles are available. As the button in the position of "X" on the Super Nintendo controller, this button is less used than the "X" and "Circle" buttons on the PlayStation controller.
Square
According to Goto, the square symbol, used as the leftmost of the four buttons, "refers to a piece of paper; I had it represent menus or documents and made it pink." This button would then make sense to summon a menu overlay on the game from which you can choose weapons, spells and other options or save the game, for example. As the position of this button was the same as that of the "Y" button on the Super Nintendo controller, perhaps the least-used button of all four on that controller, this button, like "Triangle," is located above the main "X" and "Circle" buttons.
Circle and X
Goto says "the circle and X represent 'yes' or 'no' decision-making and I made them red and blue respectively. People thought those colors were mixed up, and I had to reinforce to management that that's what I wanted." It would make sense for management to question Goto on this, because although the symbols make sense from a design perspective, the colors are reversed from what most people are used to in modern design: Red is almost universally used as a warning, alert or deterrence, such as in the case of stop signs, while blue is more subdued and welcoming. This button, along with "Circle," is one of the two most-used buttons on the PlayStation controller.
Regional Button-Mapping Problems
According to game engineer Matthew Gallant, North America and Europe typically have a different controller mapping from Japan's even though the buttons, symbols and colors are identical; specifically, "X" is used to confirm while "Circle" is used to cancel, whereas the reverse is true for Japanese games. Gallant states that while the buttons are usually remapped when ported to other regions, some games like "Final Fantasy VII" and "Metal Gear Solid" left the original button mapping, resulting in confusion for Western players.