Almost every machine in the world starts to knit the sock from the Hem and then it continues through leg, heel, foot to toe. Here is the problem as the machine just finished sock but it's still just kind of weird tube. We have to close the toe by sewing it together or just connect the opposite loops to make special flat seam. Basically we can divide machines into two big groups - Single and Double cylinders. Every machine as specific amount of needles (let's say from 84 for thick socks through 168, 200 for standard socks to 320+ for women hoisery). The other specific parameter of every machine is cylinder diameter. Basically we use narrow cylinders (let's say 3 1/2" with 120 needles) for children socks and wider diameters (4" or 4 1/2" with 84 needles) for thick men socks. This means you need to think very carefully what kind of machinery are you buying as it's not easy and it's expensive to change this parameters in future.
Most common types of machines in the world as you can produce a large variety of patterns and socks on them. Also these machines can make terry socks after some changes. This machines are ideal for both sport and basic/casual plain socks. The important parameter is how many color system they have. This justifies how many colors you can put into sock. For example, if you have 6 system machine, you can put 6+1 colors in a row. However every system has more yarnfingers, so let's say if there are 3 yarnfingers for each system, you can put 18+1 colors in your sock. Well, theoretically ... it also depends how complicated the pattern is.
As there is only single cylinder, these machines can only make plain knit and it means they cannot make true rib. However there are ways how to make false rib by the elastic or by float stitch (false filet).
“Nowdays, single cylinder machines are the most dominant type of machinery in the world.”
Double cylinders are machines with another - the second cylinder above the first one. Needles are moving from one cylinder to another in order to make genuine rib. However you cannot make patterns with many colors here. Still these machines might be able to make jacquard patterns. It means you can make patterns with few colors (2 or 3) without any cutted yarns on the inside of sock.
We also have some special machines for example - machines with so called radial needles. These machines allow us to make patterns with plenty of colors and make genuine rib and terry at the same time. Some machines are able to make knitted shoes - they are used by most popular fashion brands in the world.