The difference between absolute and fixed happens when thier is more than 1 dpeth level of hierarchy.
Both are fixed values, but absolute is relative to its container (parent) and fixed is relative to its root (screen / page)
So, to best fit all cases we use
aboslute in most cases when it's not explicitly specified.
In general, you should be using fixed for global element that is always on top of all layers, for example on Bottom navigation.