Feed slots 1 and 2 - what do they do?

[Volator]

For the control settings in CA there are keys "1" and "2" that toggle "Feed slot 1" and "Feed slot 2". What do they do? 

[Shadow KT]

If you have some kind of an autocannon for example, like the 25mm Bushmaster or the 30mm on the BMP-2/3, which has double feed, you can switch between the two feeds... AP and HE