A spline is the long, rounded, flexible piece of material you will see in the rails of your sling patio or pool furniture, awnings or slide out covers on RVs which insert in the loop sewn on each side of the fabric sling.
This is used to keep the fabric securely attached across the two rails. When cut, spline is called a sling rod and is inserted into the sling pocket to keep the sling from pulling out of the sling rail.
The spline must be large enough to keep from slipping through the opening slot running along the rails yet not too big to fit well in the ends of the rails for easier installation.
Most quality patio or pool sling furniture such as Winston, Woodard, Tropitone, Mallin, Carter Grandle, Brown Jordan, Agio, Innova, Telescope, Lyon Shaw, is designed to use spline material to install the fabric replacement slings on the frame.
There are some exceptions like Homecrest or Samsonite which use two layers of material on some of their furniture to slide the slings over an internal frame.