Sweet & Sour is an ingredient which has fallen from favour in recent years in the bar industry. A mix of citrus juices and sugar (generally lemon based but often included lime and occasionally orange juices), used in drinks such as the Long Island Iced Tea.
While using Sweet & Sour can be useful in high volume bars to keep drinks flowing fast, using lemon or lime juice and sugar syrup gives the bartender far greater control over the balance of drinks, being able to find tune the sweetness based on the other ingredients.
Sweet & Sour is sold premixed by companies like finest call (in a handy store & pour style bottle) and also as powder mixes which can be mixed with water to make large quantities of the mix with long shelf lives.
The mix also often includes a foaming compound to give a nice stable foam to the head of the drink when shaken, which now is more often achieved using egg white as in many traditional recipes.