The Triple Constraints

"Triple Constraint’ , you can also say the Project Management Triangle or The Iron Triangle.
The key attributes of the Triple Constraint are Scope, Cost and Time.

Scope - Project scope deals with the specific requirements or tasks necessary to complete the project. Scope is important to manage on any project, because if you can’t control the scope of the project, you’re not likely to deliver it on time or under budget. When managing scope it’s critical that you prioritize your tasks, enabling you to plan and assign resources effectively.

Cost- The financial constraints of a project, It is also known as the project budget.

Time- The schedule for the project to reach completion.

You cannot change one of the constraints without affecting either one or both of the other constraints.

Specifications can have different meanings (in PMI standards and guides as well as in life), it is in my view rather a document that groups a specific type of artifacts: requirements specification, test specification, design specification, business procedure specification etc. - and yes, it is related to product scope in my experience.

Requirements are defined by stakeholders, e.g. a customer and elicited by the BA/PM during the 'collect requirements' process in PMBoK Guide.

The 'Define Scope' process then selects these requirement that will be implemented. Not all requirements found will be implemented, there will be contradicting requirements, inconsistencies and the sum of possible requirements might not be implementable with the given schedule and cost constraints. Hence I never saw a project that implemented all requirements.

In procurement this is represented by the RfP includung a SOW (statement of work), produced by the buyer, and the proposal by the seller.

Requirements are the wish of the sponsor organization or customer, the scope statement is what they are promised by the project manager. In Germany, we distinguish this wish/promise pair by Lastenheft / Pflichtenheft.

Thomas. In English the wish/promise pair are called Requirements and Objectives (R&O).

Requirement definitions include the terms "must" or "shall" and are not optional. Objectives include the term "should" and are desirable, but not absolutely necessary.