'X' refers to the products or services that will actually be provided by the project.  In other words, this defines what will be provided to stakeholders in terms of systems and outcomes.  On the other hand, 'Y' identifies the tasks and elements that will need to be achieved, to actually supply the 'X' effectively.

'Y' refers to a document that explains how project requirements will be analysed, documented and managed.  On the other hand, 'X' is typically a tabular listing of the requirements, and the status of those requirements.

'X' refers to a document that explains how project requirements will be analysed, documented and managed.  On the other hand, 'Y' is typically a tabular listing of the requirements, and the status of those requirements.

'Y' refers to the products or services that will actually be provided by the project.  In other words, this defines what will be provided to stakeholders in terms of systems and outcomes.  On the other hand, 'X' identifies the tasks and elements that will need to be achieved, to actually supply the 'Y' effectively.