Time Travel is the concept of traveling either backwards or forwards in time from the present date. The ways and rules of time travel may vary depending on the Universe, with some of them having completely different rules on how they function.
On Earth-515, time travel to the past does not alter the future, but rather creates a branching timeline that coexists with the "Sacred Timeline" (this Universe's main timeline), until one of the following happens:
- the branching timeline eventually withers away and becomes one with the Sacred Timeline once again;
- the branching timeline evolves to a point that it becomes a whole parallel Universe;
- it is pruned by the Time Variance Authority, to prevent the previous possibility from happening. This no longer happens since the TVA changed its goal from pruning branching timelines into hunting down Nathaniel Richards' variants across the timeline.
Furthermore, any time travel that does not cause changes to the timeline was always destined to happen - an example is Cable being sent as a baby to the 37th century and, as an adult, returning to the past. Cable's time travel did not create any branching timelines because his travels are fixed points on the timeline. His visits to the past don't alter the future he comes from because said future was created in part because of his actions in the past.
Another example of this kind of time travel is Ben Tennyson and Rook Blonko's hunt for Maltruant in both 1770 and the creation of the Universe. As their presence in these two points in time were already destined to happen, their time travels did not create alternate timelines.
On Earth-38, time travel to the past does alter the future, but time is much more malleable in this Universe and such alterations are not immediate, being possible to undo the alterations before the new timeline is set. When a timeline is set, the previous one is erased by the own temporal nature of this Universe, and if an individual from that erased timeline survives the temporal changes, they will maintain their original memories in addition to the memories of the new timeline.
There are no such thing as branching timelines on Earth-38.