Prior to Firestorm 4.7.1 (4.6.9 & earlier), on Windows, the viewer used DirectX to see how much video memory (VRAM) the graphics card had, and that was used to set the texture memory setting. There were problems with this though, because DirectX had a bug with certain AMD cards causing the video memory to report as stupidly low, so a user would be forced down to 64MB texture memeory (for example), and suffer extreme texture thrashing.
On 4.7.1 and later, Windows now uses WMI to check the graphics memory on the card. This also has problems if you have a dummy graphics driver installed for something like logmein or joinme. It can report a stupidly low graphics memory. So, this setting allows you to choose which method to use to measure graphics memory on your card.