A while back I got a qestion about why a certain white took so many coats to show up. I made a little comparison chart to show the differences in three different whites commonly available in acrylic.

I mixed each white about 50/50 with Diox Purple just so things show up better and painted a swatch over a black line to illustrate opacity. Keep in mind that color is added here for illustration purposes.

The swatch on the left is using Zinc White (PW 4). Zinc White is a transparent white and you can paint 'til you fall off your chair and it won't give you the coverage as in the next example.

The center swatch is using Titanium White (PW 6). This is an opaque white, and you can see that the black line is obliterated. This is a good choice if you want coverage.

The swatch on the right is using a type of white known as Tint White or as Mixing White, depending on the brand. It is generally a combination of two white pigments - one opaque and one transparent. You can see that the black line is partially obscured.

Uses?
Zinc White - Great on its own for glazes to adjust value without obscuring the underlying detail too much. I love this white for making areas appear soft while providing subtle value changes. Just a thin glaze over detailing provides just enough coverage.
Another advantage is for making tints from other transparent/translucent pigments (Diox Purple, Ultramarine Blue as examples). Zinc White maintains most of the transparent properties of these types of pigments (as shown on the chart). Adding Zinc White to tint opaque pigments will cut the opacity somewhat (depending on how much is added).

Titanium White - Great for accenting where coverage is desired. White gesso is another option. I personally don't like to dilute it for glazing because I think it gives a chalky appearance. It will also obscure underlying detail more than Zinc White - not necessarily a bad thing if that's the effect you're after. Just one more option. Using Titanium white to make tints using transparent/translucent pigments will make these tints more opaque (as shown on the chart).

Tint White or Mixing White - I don't use this. I just bought this out of curiosity. If I needed something like this, I figure I can make it myself from Titanium White/Zinc White and be able to control the translucent properties by adjusting the amounts of each white.