Don't be fooled, slide rules are really easy to use. It took me less than a minute to learn how to do multiplication, and I wasn't an adult yet.
The A/B/C/D names are from Amédée Mannheim, who in 1851 designed the "modern" slide rule & give those scales those names. In practice, you normally just use the C & D scales when you want to multiply. Here's a history of the "cursor" on a slide rule, that also provides a clear history of the slide rule itself: https://www.nzeldes.com/HOC/Cursors.htm
I've never had a practical reason to use a slide rule over a calculator. But I've had fun with them, & that means something.
The A/B/C/D names are from Amédée Mannheim, who in 1851 designed the "modern" slide rule & give those scales those names. In practice, you normally just use the C & D scales when you want to multiply. Here's a history of the "cursor" on a slide rule, that also provides a clear history of the slide rule itself: https://www.nzeldes.com/HOC/Cursors.htm
I've never had a practical reason to use a slide rule over a calculator. But I've had fun with them, & that means something.