Currently there are only two bars with seating that serve food in the World Showcase (Rose & Crown Pub in Britain and La Cava del Tequila in Mexico). They do not serve full meals and don't take reservations, and seating at both is limited.
Everywhere else, you would likely be purchasing beverages and snacks from outdoor kiosks or from shops with no seating, and if you want to sit you will need to locate seating elsewhere in the pavilion.