I’m as new to color management as I am to contributing to forums, but it doesn’t look like anyone else has answered your questions, so I’ll give it a try.
“Ink Restriction” or “Individual Ink Restriction” refers to reducing the maximum amount of each ink that can print. For example, if a file calls for 100% cyan, the press may have the capability to deliver an excessive amount of this color. Not only is this a waste of ink, it becomes very difficult to color correct or create a profile when your individual inks are printing too heavily. This is usually the first step in the process of creating a profile. There are criteria that are used to determine what the correct setting should be. I’ll get to that.
“Ink limits” or “Total Ink Limit” refers to the combined amount of ink all 4 colors (or 6 or 8, depending on your press) will print. I’ve seen this described as a maximum of 400% or 4.0. This is the last step in the process before printing out patches for creating an ICC profile.
Being new to color management, I know of only two methods of determining Individual Ink Restrictions. One is to use density readings. The other is to use Chroma (Lch).
Using the density method, you would use a predetermined range for each ink. An example would be C: 1.4 to 1.45, M: 1.4 - 1.45, Y: 1.0 to 1.1, and K: 1.6 to 1.65. By printing out and measuring a 20 step linearization chart, you can determine what percentage of each ink hits these target readings. It may be below 60% or as high as 100%. Our Epson inks are restricted as low as 50%.
Using the Chroma method, you measure the same chart using Lch. Look for the chroma to start to drop as you measure from dark to light. I’m not sure of the correct criteria, but some inks actually gain chroma as they print lighter - to a point. If the 85% step reads at nearly the highest chroma, that’s where you would set your limit.
Sorry for the long-winded explanation. I hope it helps. Please remember, I’m very inexperienced so take what I have to say with a grain of salt and refer to your Onyx manual for more accurate information. Better yet, hire an qualified consultant if possible.