Test Card A has a lot of significance as it is the very first BBC Test Card to be transmitted. Although technically advanced for the time, this Test Card (and Test Card B) were criticized for not having sufficient testing features, and they were soon replaced by Test Card C. It was last shown 26th March 2013 as part of the BBC HD Shutdown.
Because this was the first BBC Test Card, the information on it was mainly used by engineers. The circle in the middle contains MC frequency gratings, and that was the most useful part of the card to engineers. Also the bars around it had MC frequencies.
Also, as this was the very first transmitted Test Card, it was only displayed in black and white. The first color Test Card was Test Card F.