The Work in Progress area is a reporting area that shows all invoices that are "In Progress" or "Completed", but not finalized. We won't go deeply into this because it's rather straight forward. This area is mostly for house keeping.
Many practices may have a policy to prepare invoices ahead of when they're needed. A prime example of when this is benificial is when customers might be coming to pick-up medications or other items. It's useful to create those invoices ahead of time because it will facilitate a smooth checkout later. However, sometimes the customer never shows up. There is where you can see the progress of all those types of situtaions.
It is recommended that once works in progress reach a certain age, they are dispatched to maintain data cleanliness and to ensure that piles of unpurchased inventory are not accumulating anywhere in the practice.