Revenue Recognition is a common problem in business systems. It's all about when you can actually count the money you receive on your books.
If I sell you a cup of coffee, it's a simple matter: I give you the coffee, I take your money, and I count the money to the books that nanosecond. For many things it gets complicated, however. Say you pay me a retainer to be available that year. Even if you pay me some ridiculous fee today, I may not be able to put it on my books right away because the service is to be performed over the course of a year.
One approach might be to count only one-twelfth of that fee for each month in the year.
The rules for revenue recognition are many, various, and volatile. Some are set by regulation, some by professional standards, and some by company policy. Revenue tracking ends up being a quite complex problem.
Imagine a company that sells three kinds of products: word processors, databases, and spreadsheets. According to the rules, when you sign a contract for a word processor you can book all the revenue right away. if it's a spreadsheet, you can book one-third today, one-third in sixty days, ad one-third in thirty days. If it's a database, you can book one-third today, one-third in thirty days, and one-third in sixty days. There's no basis for these rules other than imagination.