I'm replacing and old cotter crankset with a square taper one. I want to buy a 52/42 crank. I thought I had it all covered by making sure I had the appropriate spindle length and taper (JIS) in order for my crank and BB to match. But now, in the specs of the crank, it is written that it is for "8/9 speed". Another one for sale is said to be for "6/7/8 speed". I don't understand what it means and how a crank can be defined by the number of sprockets on the freewheel (or cassette).
Sheldon Brown seems to say that it's nothing but marketing (sorry, I can't post the url, I'm too junior on this forum). Here is a copy paste:

Component manufacturers like to sell you lots of new parts, even if you don't need them. This has led to much confusion as various parts are labeled as if they are incompatible with other parts even though they are not.The following parts ONLY are "speed specific":

• Indexed Shifters These need to have the correct number and spacing of detents ("clicks") to match the system they'll be used with.
• Cassettes It is really the cassette that determines how many speeds you have in back.
• Chain As you go to more sprockets on the cassette, you need a narrower chain. However, using a narrower chain with an older system rarely presents any problem.

Can anyone give me an explanation ?
Thanks!

PS: I don't know if it matters but the bike has friction shifting.