Mark partition as active?

[Crosshairs]

In disk management there is an option to "Mark partition as active"

What does this mean..and what does it do?

 

[Audioguy]

Boot partitions are always marked as active if this helps explain it. I understand it as a flag the system "sees" to denote boot capability. I may be messing up the wording a bit but I think you understand what I mean.