It's probably to indicate that programme is being viewed via Sky Box Office. Further, different viewers might see different symbols. If someone then illegally streams that content online it will give those investigating the copyright infringement a place to start in the hunt for the perpetrator.
For example, investigators might see that an illegal stream of one boxing match has got a horizontal rectangle with a circle in it. If only one in ten viewers received that symbol the investigators can narrow down their search to 10% of their audience. If they then ensure that everybody gets a different symbol (to the one they had before) for the next big fight, which is also illegally streamed, they'll then be able to narrow their hunt down to 1% of the audience. Doing the same again for the next couple of matches will find just one person in every 10,000 viewers who has received all of the symbols which have appeared on the streams. Thus, in a fairly short space of time, they can workout where the source of an illegal stream is.