Cash in advance is a term describing terms of purchase, when full payment for a good or service is due before the merchandise is shipped. This presents the least risk to a seller while having the most risk to the buyer. It is often combined with other terms such as Free On Board, which require the buyer to take possession of the merchandise as soon as it is loaded onto transportation, meaning the buyer is out of luck if something happens to the shipment en route. In actual daily business these sort of terms are extremely rare unless the goods or services are of phenomenal value and high fragility.
A constraint is any operating condition that puts a limit on the ability of a business to conduct it's operations.
A company with $5000 on hand and incomes of $3000 a month has a constraint of $8000. That means, if the terms of an economic exchange (buying equipment, etc) require terms that are cash-in-advance, then the limit that the company can actually obtain is $8000.
In these modelling theories, CIAC tends to show that up-front restrictions artificially limit the ability of companies to maintain positive inventory levels while reducing capital investment. They also inhibit real wealth in terms of cash on hand while elevating the likelihood of using junk bonds as instruments of solvency, a dangerous premise.