"Servings" are the string that is wound onto the bowstring, at the center portion of the string. It keeps the trigger latch from cutting the string hidden underneath. Also, the sheath wound around the string at it's center.
Halo serving is the toughest material known to me, to serve crossbow strings. It will outwear many of the other materials on the market. It is tops for abrasion resistance and wear.
Look closely at the Exocet 200 and Pheonix models. Both are excellent crossbows with a very good trigger. Dead solid reliable. I actually like the Pheonix the best for all day shooting. It is much easier to cock and it won't beat you up like the 200lb and 225lb models. Largest moose taken in Canada last year was taken with a Pheonix.
When you are ready to buy give me a call. I've got a couple closeouts at great prices. (2009 models)