Organizations represent the business entities that have the budget to do advertising.
If you're an agency, the organizations are your clients.
If you're a company, the organizations depends on the nature of your business.
For hospitality groups, organizations are hotels.
For retail store operators, organizations are retail stores.
For car dealership operators, organizations are dealerships.
For education networks, organizations are schools.
For health networks, organizations are hospitals.
Organizations must not be bound by time and be as permanent as possible throughout time to facilitate budget/expense comparison.
Hotel ABC spent X$ in FY2017, spent Y$ in FY2018 so far, and is planning Z$ in FY2019.
Of course, you may add/remove organizations occasionally to reflect your business structure changes.
New client, Sell an hotel, etc.