We eat in the סוכה in commemoration of the exodus from Egypt. it represents either the ענני הכבוד or virtual סוכות. some people eat, sleep, and do all the other activities in the סוכח as well.
it is not necessary to eat snacks witch do not consist of a מזונות the סוכח such as fruits veggies candy drinks saladas or chips. some people have the custom to be strict about this as well and they eat and drink everything in the לישב בסוכח .סוכח
is only said when eating המוציא or מזונות. it is said after the regular required blessing and before we eat the food. if you forgot לישב בסוכח in the begging of your meal, you may say it at any time during the meal.