Half Side-by-Side (SBS), means the left and right views of a 3D video are subsampled at half resolution and you get a backwards compatible full frame.
Example: Say, each view is 1920×1080
subsample to 960 x 1080, put them side by side and now you have a 3D 1920×1080 frame.
Full SBS, means you transmit both views at full resolution; better quality, but bigger file.
https://www.quora.com/3D-Movies-What-is-the-difference-between-half-SBS-and-full-SBS