Chinese style or Japanese style dark/light soy sauce? They might be called the same thing but they aren't always the same. Also, DO NOT confuse "low sodium" soy sauce with "light" soy sauce. They are not the same thing.
Chinese styles:
Dark soy sauce (lao-chou): It is a lot darker in color. It got a richer fermented soy bean taste (which is not a bad thing). It is not saltier than regular soy sauce and some varieties could even taste a little sweet. A small amount of this soy sauce will give you very dark colors and rich soy bean taste; regular soy sauce can not give you the color and the taste without making the dish overly salty. Dark soy sauce and regular soy sauce were usually used together.
Light soy sauce (sheng-chou): It is light in color. The fermented soy bean taste is a little bit lighter. This soy sauce is NOT less salty than regular soy sauce. You use this soy sauce when you don't want your dish to look overly dark.
Japanese styles:
Dark soy sauce (koikuchi shoyu): This is just regular soy sauce commonly referred as shoyu.
Extra dark soy sauce (tamari shoyu): This soy sauce is very similar to Chinese dark soy sauce expect Japanese tamari is a little saltier.
Light soy sauce (usukuchi shoyu): This soy sauce is light in color and noticeably saltier than regular soy sauce. Japanese light soy sauce and Chinese light soy sauce are not that interchangeable.
Again, you do not want to mix these with low sodium varieties.