Metals can only be filtered out when they have precipitated out of solution as larger particles capable of being captured by a filter.
In the case of iron, ferrous iron (Fe2+) is soluble in water at normal pool pH values. Therefore it stays in solution. When ferrous iron is oxidized to ferric iron (Fe3+), the ferric iron is not stable in solution (unless the pH is extremely low) and so it will precipitate out as fine ferric oxide and ferric hydroxide particulates. Those compounds impart a yellow-brown color to water. When ferric iron is present, it can be physically filtered out of water.
However, it is impossible to control scale by simply raising pH and/or adding an oxidizer like chlorine as the metal can form a stain on surfaces just as readily as it will form suspended particulates. Ascorbic acid acts as a reducing agent which converts ferric iron back to its more soluble form, ferrous iron.
Sequestrants can have complex behaviors depending on their formulations. Some, like EDTA and HEDP, work to simply hold metal ions in solution as a stable complex formation. Some polymeric sequestrants work by creating complex formations with metal ions and calcium ions and then those larger complexes come out of solution as precipitates in the filter taking the metal ion out of the water with backwashing. Typically, once a metal ion is sequestered, then it can't be filtered.
