Ask whoever recommended it.

My guess is that it is "low" because it is for a basement bathroom, the drain line should be made of "iron", and "septic system" because it happens.

Look at this document:

https://www.ci.faribault.mn.us/DocumentCenter/View/1947/Tips-for-a-Healthy-Septic-System

This is probably why you were offered a low-iron septic option:

Iron Filters: These are very similar to water softeners in that there discharge water is not sewage and does not need to go to the septic system. The back flush of an iron filter removes the iron as a solid which can cause the soil surface to plug in your drainfield. This can lead to premature failure of your septic system.

My guess is that low-iron septic system option would have the iron filter backflush discharged separately and not into the septic drain.

Here's some info on septic systems. https://www.ci.faribault.mn.us/DocumentCenter/View/1947/Tips-for-a-Healthy-Septic-System Iron is mentioned as the back flush of an iron filter. It removes the iron as a solid. Which can plug the soil of your field. So ask about it.

If you want a basement bathroom they may be talking about the level of the septic system to work without a pump for that area. Which could apply to laundry.