Menu Periods - What is the difference between a menu period and a service period?

This is a really good question, there are a couple key difference between the two that are very important to know.

Service periods are the rules by which your meal plans abide by.  They would be configured in FIIT and they apply constraints to meal plans such as how many meal units/DCB you can use per period. For example, if the meal plan rule is that students can only use one meal per service period, then when they come in and eat during lunch they will only be able to spend another meal unit once the next service period, dinner for example, rolls around.

Menu Periods are the menus for each of your service periods. They would be configured in Nōwn and they are the menus that show for each service period. For example, maybe you want breakfast to be a different menu than lunch, menu periods allow you to do that. They can be the same as your service periods but don't have to be.