What else could the ERD below have added to it? Based on the following case: (Use same layout)

CQ constructions Pty Ltd (CQC) is a successful construction company that implements many residential unit construction projects in Australia. Each of those construction projects will have a name, project start date and number of towers that will be constructed in that project. Each tower is expected to have many floors and each of those floors can have many units in it. However, all floors in the same tower will have the same layout; same type and respective number of units. The units can be of single bedroom unit, double bedroom unit or three bedroom units only. If a floor, in a tower, contains two numbers of three bedroom units and four numbers of two bedroom units, then all the other floors in the same tower should also have the same number of two and three bedroom units.

If a customer would like to purchase a unit, he/she has to execute a purchase agreement with CQC and agree to pay the installments as specified in the purchase contract. Generally a purchase contract will specify the details of many mile-stones related to construction such as completion of excavation, completion of foundation, etc. At each of the mile-stone achievement, an installment payment is required from the customer. The details of those installment payments have to be recorded for auditing purpose.