Hmmm... if I recall correctly, the principle of an overarching timeline comes when... eh, let me make an analogy:
Timeline A encompasses Timeline B, and Timeline A is been said to have its own time axis separate from Timeline B. Timeline A, as depicted as being a superior structure, contains Timeline A, and since Timeline B acts like a subset of Timeline A, manipulating the time and time travelling in Timeline A will also affect Timeline B as a byproduct. However, if manipulating the time in Timeline B and time travelling only in Timeline B, Timeline A won't be remotely affected at all. Hence, Timeline A is a hyper-timeline / overarching timeline and, therefore has a second time axis orthogonal to the main time axis that is inter-governed and parallelly aligned with Timeline B.