In order to understand the answer, you need to be familar with the body fluid compartments. Namely, Intracellular (ICF) and Extracellular (ECF). ICF is the fluid inside the cells. Look at it as this: ICF are the cells outside the blood vessels. ECF has subsets, it comprises the fluid surrounding the cells which are located outside the vessels - this is called Interstitial fluid and the blood vessel itself is called intravascular.
If you have someone whose cells are dehydrated, you need to give an IVF which will hydrate the cells, namely a HYPOTONIC fluid as this fluid will go from ECF to ICF.
In contrast, if you have someone who just returned from surgery and who has third space fluid shifts, a HYPERTONIC fluid may be appropriate as fluid will leave the ICF and interstitial areas and come into the Intravascular ECF.
Thyen, one with fluid loss from N/V, an isotonic solution may be appropriate.
Hope this helps.