Dragan is pretty spot on with his explanation. The durability is in reference to how long the string will last before breakage.
The playability duration is an especially important factor when considering polyester strings because of their nature. While the durability may keep the strings in your racquet longer, the playability duration is the expected life of the "feel" of the strings. Even though you can keep RPM in your racquet for a long time before it breaks, it doesn't mean that the expected performance qualities of the string will still be there after the playability duration has passed (which is why a lot of people can become disappointed with Polys and Co-Polys after roughly 2 weeks).
If your strings lose tension before you break them, then continuing to play with the strings can have ill effects (elbow pain, no power, etc...). If you break polyester strings before they lose tension and feel, then you may need to consider a more durable string.
Sorry for the long-winded response, and I hope this is helpful.
TW Customer Service